DD is 23 and still is sleeping in her twin loft bed. She likes having 2 dressers under the bed. She has the smallest bedroom in the house (10 x 9), so space is tight. When her brother moved out 2 years ago she was offered his bigger room (10 x 16) with a full size bed, even offered to buy a new mattress, she likes her room better. BTW, she is 5 ' 11"
I slept in a twin bed from age 3 until just before my 25th birthday when I got married. I'm 6' 2" and was plenty comfortable.