That's complicated, considering they're reincarnated and were born centuries ago, even before the Renaissance artists their reincarnations were named after. In mental age they were teenagers when they were killed in their human lives, though not all the same age. But since reincarnation, they can (theoretically) keep maturing now.
Yes, but the specific mental ages of each of them are murky. We know that in their human lives, the blue son was older than the red son, who was older than the purple son, who was older than the orange son, but all were teenagers when they died. Assuming none of them are twins and there were no premature births (none of which I can say for certain, but for the sake of this thought experiment), that would mean that each of their births were separated from one another by at least 9 months, making human Leo at minimum 27 months (2 years and 3 months) older than human Mikey. So if there have already been three Christmases since they mutated plus at least several months when Raph was still living on his own, then right now Mikey's minimum age is 16 or 17, which could likely make Leo already 18, 19, 20 or older. Their childhoods are probably already coming to an end, which is just as well because that's what's eventually supposed to happen.
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? Enemies Old, Enemies New: Prologue
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 tall near-adult-looking teenagers in TMNT #85.