Board Thread:Questions and Answers/@comment-33790467-20190220205255/@comment-995426-20190222140650

Here, I think it's easiest to guess relative ages of children when they're all very young. The red son is smaller than the purple son, despite being older. But the clearest comparison is between the blue son, and his orange brother he's carrying in his arms. The orange son appears to have still been a toddler at most, so how old do you think the blue son is in this picture?

I should caution though that art like this is not always an authoritative representation, especially as artistic rendering can vary not only between rotating comic artists, but can also be retconned across stories, even in subtle ways between stories that are otherwise close together in publication order. The Children of Saint Robert's Orphanage are good examples of this, as the universally preteen children depicted in Kingdom of Rats, part 4 magically include some near-adult teenagers in TMNT #85.