I understand what you mean, but the character design says otherwise. From the viewer's perspective, David (the dog) looks too old for the protagonist, both because of his breed and his coat. I know it's a good game, but a redesign wouldn't hurt, or perhaps they could change his breed.
You do have a good point here, the first thing I thought when I saw this vn was exactly the same thing you are writing here
Sooooo, I found out that, if you want a furry to look old, u can give him a moustache in the snout, somehow, that makes it look old, and, you can take it further if you paint the snout (and if you want, the jaw too) white/grey
Now, does David share these points?
Yep, he does
Here's a little example of the point I'm trying to make here :D

Also, his coat looks like something a grandpa would use trying to look cool with the young xD
Heck, I think it would be really funny if in the vn David had this problem when he meets new people
Eh, I don't think that's a problem, Ranzo from Dawntide (awesome and marvelous vn, almost everyone should check it out) has the same type of hair, I even think David and Ranzo are the same breed (Schnauzer)
Here's an image of the dawg in question:

Idk how, when, where, and why, but, Ranzo shares some points in the "why does a furry look old?" thing we got here, and somehow to me, seems like a 20-30 year old
I would say the color in the snout is the one making us think he's old, but, yeah, his hair just grows like that, he could just go to a barbers shop and everything would be done, David just doesn't do it because (reason goes here)
Even so, I feel like in a furry world type of setting, it would make a good plot point, like, maybe it would be a little silly issue the character has to bear or, a deeper insecurity eating the character little by little..
Idk, I think I should just make a vn by this point, I have a lot of ideas and it sounds fun, anyways, see ya, thx for reading and have a good day (or night), stay safe out there :D