As I understand it, it's all about probabilities. So it is more likely that an older player drops his skills fast, but it can happen to a younger guy too if he's unlucky. However, players under 31 or so practically cannot drop a skill (stamina is a different case, everyone is likely to drop it about once a season if not trained). As said before, in addition to age, skillset might have an impact on the dropping (well-rounded less likely to drop) and possibly there is a random factor behind all this too. So it is normal if your player drops some skill two levels in one week and after that doesn't drop for a season.