Yes, the yellowing is due to the age of the glue and I really don't know how fixable it would be. Just be extremely careful!
Also, just throwing my two cents here, I really don't think that toothpaste is a very good idea. You know how when you bite down when you brush your teeth and you can feel a little crunch? That's the abrasion that's added to get the gunk off of your teeth with the friction. I would think that the little crunchy things wouldn't be very good for the flocking, and might take some of it off.
I guess it's worth a try if you let it sit for a while on the pony without actually rubbing it, but just be careful.