Does your tulpa age / change appearance as they get older?

I was thinking about this and I don't think I ever want my Tristan to get old. I don't see why he would ever need to. So I can just visualize him the same way every time, and that should work... I was curious if your tulpas age? And in general, do tulpas have one set physical appearance, or can they shapeshift to look like different people or stuff like that?