You're never too old to have a stuffed animal friend!
I have an ancient stuffed bear who goes with me whenever I travel...she's always near me when I go to sleep (unless I'm going camping and she might get dirty).
I've never made anything for a Build-a-Bear, but I just finished making a sweater for my bear. Do you get clothes for your bear? You could start by measuring the bear's clothes and trying to design a pattern from there based on human patterns you like. I measured my bear's dress and made the whole back stockenette, and the front all Grecian Plait (off the free patterns section here).
The thing that threw me off the most is that my bear (and many other stuffed animals) has a bit of a permanent slouch. When I got to the top of the back, I had to reduce the back and knit more in the center, sort of like shaping a sweater sleeve.