My son wanted to ride until he was 6

. At that age, we just had one to walk to and from MK (saved my poor husband's back from having to carry him back to the hotel at midnight). When he was 4 and 5, he rode in it quite a bit and napped in it a lot (and this is a child who stopped all naps at 18 months).
We walk a minimum of five miles per day (usually closer to seven) when at WDW - that is a lot of wear and tear on those little growing muscles (and on our muscles if we have to carry them).