Other than physically being able to train, I'd say almost the only obstacle. There are about three people I see on my regular routes, almost every day. All (from appearances) from completely different circumstances, physical abilities, ages, and perhaps purposes. All including myself - I am older than they - have improved strikingly over the last 2-3 years. I think the only common factors have to do with determination and mental toughness. They have it, and the dozens or hundreds others I see come and go do not and will never make gains or get much out of it regardless of age or physical ability.