Youth bed mattresses are the same size as a crib mattress. Most cribs convert to a youth bed these days & I have made "youth bed" quilts but I knew they wouldn't be in that size bed for long. I made the quilt larger than I would for a crib for the drop on the sides & I figured the kids could use it later for watching TV, etc. I'd say by about the age of 4, they'd probably outgrow a youth bed.
A crib/youth mattress is 27" X 51" X 4-6" thick.