For one thing, if mutants are supposed to be similar or the same as Yokai, they must have similar life spans. Big Mama and Draxum are both hundreds of years old and they both aged very well. Now, we don't have any examples for how mutants should age besides Splinter, but I decided it makes more sense for him to age like a Yokai than to age like a human. Therefore, I kept him younger. Also, no amount of aging will make a person lose 2-3 feet of height, but that's just a personal pet peeve of mine.
That's mostly it, but some other things. The one on the right is purposely trying to cover up his belly with his arm and his clothing choices to seem like he hasn't lost himself. His hair is greasy and worn down because he's depressed and hasn't been taking care of himself. In general, I tried to draw him awkward and ashamed of himself.
As for the one on the left ... it's surprisingly hard to draw a pompadour on a rat, and that's all I'll say. The design is supposed to resemble Lou Jitsu and display a lot of the confidence that he's lost in his time as Splinter.
Edit: since it was asked about, yes, he does have fat on him, he's just hiding it with his clothing and arms. This is intentional. You can still see it in the belly/arms.