Answer by Raakhee V. Menon:
The ages of the various characters of our epics have been a matter of great debate since forever.
There are some popular versions like how it is said that Abhimanyu was 16 years old when he was killed on the battle-field, which would imply that the Pandavas (and Krishna) were probably in their early 40s when the battle at Kurukshetra happened. The time frames are very vague owing to several overlaps. One must take into consideration some factors while thinking about the age of the characters…
Firstly, the Mahabharata is a compilation of works by not just one author, but several authors. So there could be some overlooking/conflict w.r.t. birth dates, etc.
It is also not clearly known how long the Pandavas took to create the city of Indraprastha. After Dhritarashtra gives Khandavaprastha to the Pandavas, they go there with Krishna and Balarama and set about constructing a city on what was basically a desert.
‘Agreeing to what Dhritarashtra said, those bulls among men worshipping the king set out from Hastinapura. And content with half the kingdom, they removed to Khandavaprastha, which was in unreclaimed desert. Then those heroes of unfading splendour, viz., the Pandavas, with Krishna at their head, arriving there, beautified the place and made it a second heaven. And those mighty car-warriors, selecting with Dwaipayana’s assistance a sacred and auspicious region, performed certain propitiatory ceremonies and measured out a piece of land for their city. Then surrounded by a trench wide as the sea and by walls reaching high up to the heavens and white as the fleecy clouds or the rays of the moon, that foremost of cities looked resplendent like Bhogavati (the capital of the nether kingdom) decked with the Nagas. And it stood adorned with palatial mansions and numerous gates, each furnished with a couple of panels resembling the out-stretched wings of Garuda. And it was protected with gateways looking like the clouds and high as the Mandara mountains. And well-furnished with numerous weapons of attack the missiles of the foes could not make slightest impression on them. And they were almost covered with darts and other missiles like double-tongued snakes. The turrets along the walls were filled with armed men in course of training; and the walls were lined with numerous warriors along their whole length. And there were thousands of sharp hooks and Sataghnis (machines slaying a century of warriors) and numerous other machines on the battlements. There were also large iron wheels planted on them. And with all these was that foremost of cities adorned. The streets were all wide and laid out excellently; and there was no fear in them of accident. And decked with innumerable mansions, the city became like unto Amaravati and came to be called Indraprastha (like unto Indra’s city). In a delightful and auspicious part of the city rose the palace of the Pandavas filled with every kind of wealth and like unto the mansion of the celestial treasurer (Kuvera) himself. And it looked like a mass of clouds charged with lightning.
“When the city was built, there came, O king, numerous Brahmanas well-acquainted with all the Vedas and conversant with every language, wishing to dwell there. And there came also unto that town numerous merchants from every direction, in the hope of earning wealth. There also came numerous persons well-skilled in all the arts, wishing to take up their abode there. And around the city were laid out many delightful gardens adorned with numerous trees bearing both fruits and flowers. There were Amras (mango trees) and Amaratakas, and Kadamvas and Asokas, and Champakas; and Punnagas and Nagas and Lakuchas and Panasas; and Salas and Talas (palm trees) and Tamalas and Vakulas, and Ketakas with their fragrant loads; beautiful and blossoming and grand Amalakas with branches bent down with the weight of fruits and Lodhras and blossoming Ankolas; and Jamvus (blackberry trees) and Patalas and Kunjakas and Atimuktas; and Karaviras and Parijatas and numerous other kinds of trees always adorned with flowers and fruits and alive with feathery creatures of various species. And those verdant groves always resounded with the notes of maddened peacocks and Kokilas (blackbirds). And there were various pleasure-houses, bright as mirrors, and numerous bowers of creepers, and charming and artificial hillocks, and many lakes full to the brim of crystal water, and delightful tanks fragrant with lotuses and lilies and adorned with swans and ducks and chakravakas (brahminy ducks). And there were many delicious pools overgrown with fine aquatic plants. And there were also diverse ponds of great beauty and large dimension. And, O king, the joy of the Pandavas increased from day to day, in consequence of their residence in that large kingdom that was peopled with pious men.
If you look at the scale of things, it’s kinda evident that the city was not anything less than magnificent. Assuming that no magic was involved, the construction period would have been lengthy, taking into consideration the ancient construction techniques that were almost all done manually.
In Book 1 of Adi-Parva, there is a line that talks about their time at Indraprastha…
Vaisampayana said, ‘Those scorchers of foes, the Pandavas, having obtained their kingdom, at the command of Dhritarashtra, passed their days in joy and happiness at Khandavaprastha with Krishna. And Yudhishthira. endued with great energy and ever adhering to truth, having obtained the sovereignty, virtuously ruled the land, assisted by his brothers. And the sons of Pandu, endued with great wisdom and devoted to truth and virtue, having vanquished all their foes, continued to live there in great happiness. And those bulls among men, seated on royal seats of great value, used to discharge all the duties of government. 
It talks of a certain period that they spent at Indraprastha. But it does not mention for how long. For all we know, it could have been several years.
Abhimanyu was born after Arjuna returned to Indraprastha from his twelve-year exile, along with his new wife Subhadra. Draupadi’s sons were also born at around the same time as Abhimanyu, with a gap of one year between each Upapandava.
The young princes all grew up together in Indraprastha and were trained in the scriptures by their guru, and in warfare by Arjuna. It is also written that they became warriors, which means that they were not exactly young children before the Pandavas lost everything in the game of dice. They were (in all probability) in their teens, as that is the period that a prince would have needed in order to complete his formal studies.
It is very clearly written in the text that that the Maya-Sabha construction, the Rajasuya Yagna, the invite to Hastinapura, the fated ‘Game of Dice’ and the 13-year exile of the Pandavas & Draupadi happened after this. This makes the age of the princes to be closer to their 30s when the Mahabharata battle took place. It could, of course, be more. I’m just considering a very tight time-frame.
How could Abhimanyu or sons of Draupadi be around 16 years old during the war when it was said that Arjuna himself trained them in weapons, Dhoumya trained them in Veda, and the sacred-thread ceremony was also done even before the Rajasuya Sacrifice?