While you may be right by claiming that age is not definitive in determining ceiling, it is still fair to speculate that younger players in general, hold greater capacity to improve. They are generally seen to be heading towards a peak of their powers, acknowledged to be around mid to late twenties. As you say, not all, but most would fall into that category, as it is considered the period when experience gained up until that point, allows a maximising of ability to produce a consistently high level, viewed respectively of course.
Bale may or may not have more to show us in terms of ability, but at his age he surely has capacity to learn more about how best to use his ability to find a consistently high level of performance.
Similar to CR really. In my view he has not really shown anything new in Madrid in terms of ability, that we have not already seen during his time here. But his consistency levels are off the charts, and you have to attribute the maintaining of such high standards to added experience combined with the ability we already knew about.
Drogba is a good example of the point you are making, but Owen is not so good imo. Owen was one of the most feared strikers around, but it was injury not age that was indicative of his ceiling. Injury robbed him of the generally accepted 'peak' we are discussing, so not really fair to simply suggest he didn't improve. Had he kept the pace and movement that had taken him to the top, how could anyone reasonably suggest he couldn't have stayed there?