I can imagine that to avoid being sued by specific individuals who might be denied those seats for being incapable - since people love to take offence at any imagined slight, then sue for it, especially if they see a company as the defendent, the airline in question might have decided on an arbitrary age cutoff which would ensure getting most of the age-incapables in the net without them having to tell someone they can't sit there due to a perceived incapability, then getting sued.