How old Bruce Wayne in each season of Gotham? We know he was 12 when his parents died but what about the rest of the seasons?
While not said explicitly, he is 12 in the pilot, and by season 4, we see a birthday cake for him in S4E18 which has either 17 or 18 candles on it (I had trouble seeing them clearly) which we can assume means he is 17/18 by the end of season 4.
Filling the gaps this is my rough guess:
And of course the final episode of season 5 there is a 10 year time skip, making him around 28 years old by the very end.
Here is an image from a reddit post in which they counted out the candles on the cake from Bruce’s S4 birthday.
As @Paul D. Waite said, he is 12 in the pilot episode, approaching 13 by the end of S1.
Now, i once read a threat in reddit (posted below) that i could not agree more with, where someone said:
Time is a little nebulous on the show
Which makes every answer come with a degree of uncertainty, as it is not confirmed nor denied.
I tend to think that they try and keep it as close to David Mazouz's age while filming, so my guess would be that he is 14/15 in S2, 16 in S4 and in S5 it can be narrowed down to 17 approacing 18, as he is old enough to legally drive (Young bruce is following the law :P), yet not old enough to live independently.