Reincarnation of a Sotapanna person (Stream Enterer) When the life of a Sotapanna ends, let's say he/she reincarnates as a human.

Does that mean, the reincarnated is capable of demonstrating the qualities of a Sotapanna since a very young age?

Or, does it take years of practice (again) to regain Sotapanna qualities ? 

And how long roughly does it take ?

And is there anyway to tell if a kid attained Sotapanna in his/her past life ?
I'm answering this with some experience I know similar to this.

It's says that 7years is the minimum age that it would take to understand Dhamma. I haven't seen a reference for this but it is said by many people. An example for this is prince Rahula become a monk when he was at age of 7years.

Anyway according to some stories I've heard present day, They no need to practice anything, they them selves starts to realise that there's a problem in the things they do. That's because they have the right view. So that right view starts to conflict with the view of the other people (puthujjana). So with their understanding they have started to search for Dhamma. And they have come and stopped at some monks. And these monks' view is not accepted by the majority of the people (same as in this forum). But those stream entrants people say that it is what they have looked for.


  How long does it take?


Can't give an exact answer to that. But as mentioned earlier it's said that minimum age is 7. With the experience it can go up to 20, 30 as well.

Only way to tell about past life is through pubbe-nivās-ānussati-ñāṇā (which is used to see the past lives) which can be obtained by meditation, or by asking buddha. But a rough idea can be get by asking the right view

With metta..!