Most jackpots limit the heifer age to 30 months of age. Majors require a heifer to have a calf by side at 28 months of age and can usually show until the calf is 6 months old but each TX major is different.
For Houston (others are probably the same) - Max age for a kid is 18 as of Aug 31 (the start of the school year). I.e. a kid could turn 19 if he had a birthday between Aug 31 and the March show as long as they were still in high school and enrolled in 4-h or FFA.