I'm 6'1 and don't have a problem finding clothes that are long enough. But when I was younger (I'm 30) I always found that cheaper clothes like forever 21 were always too short but places like gap, banana republic,
Nordstrom, jcrew I never have a problem. I only shop sales so I don't find those places that expensive and fit much better. But shirts don't tend to be a problem so I can find shirts anywhere and than find I have to splurge more for jeans , skirts etc.
I know you said those places were too mature for you but I find that they have things that are youngish if you look. I just bought a bunch of gap dresses that are cute and they come in tall.
Sometimes when we are tall we just have to realize our limitations. There are some things I just can't buy so I have to avoid but there are great things about being tall too. I'm sure the same goes with being short. So maybe get your tops from those younger cheaper places but on the things that are hard to find you might just have to splurge and go to other places. J crew and banana might be too mature for you but gap is a happy medium I feel. I never shopped at gap and recently went online there and I can't believe how much cute stuff they had and if you buy online there is almost always the tall option.