Why do all the charicters from the firs lion king look so diferent in the the second?

Probably because they were made sa pamamagitan ng different people. Simba's Pride was created in Australia, sa pamamagitan ng animators who hadn't worked on the original film. The new animators and supervising animators could look at the first film and study how the characters were drawn, but it didn't necessarily mean they were able to copy them exact. Here's an interview from a magazine you might be interested in:

link

Because they became older?
And lets not forget that they were made sa pamamagitan ng a completely different crew as well.
So i think there can be some differences between the movies.

they were older and different people started the 2 one

Probably because they were made sa pamamagitan ng different people and the fact that they were getting older.

Because they are grown up

The creators of TLK 2 were different animators than the ones for the original film. It's very clear that every Disney film sequel looks different such as Aladdin/Return of Jafar, Mulan/Mulan ll and many more. It's shows why TLK 2 looks somewhat different is because every Disney sequel has different animators than the ones for the first films.