1. from a common perspective, its just intuitive that you wouldn't be able to walk by age 50 even though this is not true, it just makes sense to them so they say it.

2. they look down on us and don't like that we are making moves to be on their level now. tallness allows people to feel a free sense of superiority and this is an equalizer that threatens their sad lives where their only actual accomplishment is being born with better genes... obviously not all tall people are like this or even do it consciously... but it is there and there are more of these people than you would like to think