Unlike Forever 21, H&M is great because there are many variety of pieces appropriate for all ages. In the men's section, there are clothes for both men, teens, and boys. In addition, same goes for women. It's like a mini department store.
I think you should wear what you want... And they sell children's clothes so I see it like if they are marketing to adults and children ...they must want parents to shop there too....I'm 30 and I and my 7 yr old both shop here!!
It all depends on what you are getting from there. Some of what they sell is really classic and just about anyone can wear it. Other stuff can be too trendy for some to be able to pull off, but there are more factors than just age.