Too Immature?

So, we're going to pick out backpacks today at the store for the boys, and my oldest goes for a cool Speed Racer one, and then my 5 year old picks out a Go! Diego! backpack. Now, there's nothing wrong with Diego, and for those of you who are unfamiliar, it is like Dora the Explorer's buddy. The problem is that Dora is really made for like ages 2-5 I suppose, and yes, my son is 5, but here goes the issue:

He's 5, going into a classroom of 6-8 year olds. Now I'm sure in Kindergarten he wouldn't have any problems, but if you go into a classroom of older kids who all have Batman or Speed Racer, or whatever else cool big kid cartoon hero backpack there is and then they see a kid with a preschool type "baby" backpack, aren't they gonna make fun of him?

Maybe they wouldn't, and maybe I worry for nothing, but really, I look at my oldest, and these are the age kids #2 is going in with, and well, Diego just isn't cool.

In the end I didn't have to worry anyway, because he opted for a Pokemon backpack, and I know that is OK as far as "cool" for that age range, but I guess this is only the beginning of my worries.

#2 is really immature. Well, he's not immature, he's just a 5 year old, but when it comes to intellectually, he is far above that of a 5 year old. So what do we do?

We gave the go ahead to go to 1st/2nd grade, but what is it going to do in the long run? We will wait and see what comes this next year.....