It is not at all impossible to enforce on certain countries like the United States where clear and concise birth records are kept and always available. The FIG can not force countries to keep birth records, of course, but if countries are able to take advantage of it then the rule needs to be amended or done away with. I say do away with it, always have, but that doesn't look to ever happen.
I wish it were possible to force the athletes to compete in sanctioned events from a young age so as to start a paper trail (for example, you must move up through the ranks, you must compete junior before senior). This would stop countries from suddenly having Elite level phenoms who never competed a day in their lives until they show up at Worlds/Olympics. Well, they competed, but only internally in China (or wherever they are from) and never in any event that FIG had any say over. There would be problems with this, of course, what about late blooming gymnasts who suddenly come into their own at age 17, 18 or older but were never chosen for their countries Jr. teams?
The countries could still cheat and falsify ages of gymnasts at their first junior meet, however, it would be a lot harder to do that. The countries would have to start falsifying ages farther in advance and doing so would be quite risky when you don't yet know which one will even make it to the senior level. As it is now, they only falsify the ages of gymnasts who are ready to compete Senior NOW.