This is based off of Bakugo’s age since he is the oldest. On the wiki is states in Chapters 1 and 2, Bakugo is 14, Chapters 3 and 4 he is 15, and currently he is 16. Well, Bakugo’s birthday is April (as we know). Chapters 1 and 2 are Midoriya and Bakugo in their third years in middle school. They are both 14 and they are just starting school which I gathered from All Might saying Midoriya has 10 months to train before the exams and Japanese school years last up to 11 months (April to March). Meaning from April, possibly May, (when they begin school) is when Bakugo had his first villain incident and to the exams, it would have only been to February/March. Bakugo’s 15 birthday hasn’t even passed. Then they start UA in April which would’ve been his 15th birthday. The students are all still in their first years so there is no way on full year has passed. Bakugo is still 15. It has also only been 9-10 months (shown from the Manga when it is Christmas time and after Christmas, I think after New Years, so possibly January) meaning,, everyone except Todoroki and younger have aged. Everyone older than Todoroki would be 15 and everyone young, possibly including him would be 14 still. If this was based off of when the Manga had been created, they all would’ve been 19 and 20 since it was created in 2014. Again, I could be wrong so if someone could explain the ages to me, it’d help but this was from my analysis. Sorry it was long.