Youth Mattress Size is generally 33 inches wide x 66 inches long. These are standard youth size bed measurements. Youth size mattress depths can vary from just a few inches taller. A youth size mattress Many times, actual mattress dimensions will vary by one or two inches, usually smaller. Youth beds were more popular prior to the 1980's. The advantage to a youth bed is lower price and smaller bedroom footprint. The disadvantage is the fact that children grow like weeds and they will soon outgrow your investment. With a youth bed, you need to be very careful there are no entrapment issues, including entrapment issues with regards to the guardrails. The problem with older used beds is that they may have been recalled and you can be completely unaware of this.