Ageplay to me is more about the roleplaying of a child at a certain age whereas for a little, being childlike is an innate part of their personality. Even in their "adult" mindset, they still have charactaristics that are childlike.
Being little is not about role-playing a certain age, it's about expressing those inner emotions that you can't express in your "adult" life... Vulnerability, innocence, dependence. And a little's way to do this is to act childlike. A little usually doesn't have a specific age, instead it'll vary along a spectrum. Factors like the environment they're in, the people they're with and their mood may affect how "little" a little is at any given point.
If you ask a little (assuming they don't age-play) their age, they are likely to either look at you confused, not sure what to answer, make a guess based on how they feel, or they'll tell you their chronological age. An ageplayer is much more likely to know exactly what age they are playing.
An age-player is more playing the role of a child, role-playing a child at a specific age... If you ask an age-player her/his age, then you will usually receive a concrete answer of the age they are playing.
Usually, a little won't snap in and out of their little mode straight away, it's a progression into it and a progression out of it. Ageplayers tend to be much more flexible in clicking into their childlike role and back out.
Littles make wonderful ageplayers because someone who is naturally childlike can of course pull off the role of a child seamlessly. I do not age-play, but that doesn't mean little's don't age-play... many little's are age-players.