People are not always the same emotional age - they go backwards and forwards in time all the time, and they don't realise they're doing it. Say, they hit an emotional situation. They're with someone who reminds them of a parent, and they react badly - they will instinctively go back to that time in their past where they first imprinted badly and try to relive it. If they were smart, they'd do a little re-parenting of themselves, but instead they just get caught in it and bounce around childish for a while until they come back.
This is natural, it is healthy behaviour to bounce around between ages. It's an auto-correcting system. So when you suddenly feel the need to be emotionally 14, it doesn't mean something's wrong. Your system is just auto-correcting. It's saying, right now I need a skill I should have acquired at 14 and didn't.
If you see other people do this, you can help to re-parent this piece of them so that they can get the input they need, and begin to integrate it. It is an instinctive, organic process that happens automatically, but with a little practice you can begin to do it on purpose, to shape it, make it into what you want to get the results you want.