Pandavas lived the 13th yr incognito for more than a year...might be 6 months more...because Yudhishthir didn't want to take any chance...he finished the exile period with solar & lunar calendar...
one calendar (I don't know which one) ended 6 months before the virat war...& the other one 2 weeks before virat war...
even after the war they remained incognito for 2-3 more days just to play safe...& then resurfaced...
so by this way...
Abhimanyu was 13 and a half or 14 years at the time of his marriage (if we go by ⭐️bharat)
& then the war preparation...(preparation time is stipulated it may be 61/2 months that to after numerous peace convoy visits or more than that )
so at the time of Kurukshetra war Abhi might be 16 or close to 16...