Michael made some great suggestions already, but I thought I'd pitch in a few more, hope you don't mind c:

What do you think about friendship? How many friends do you have? In our childhood we think that we have a lot of friends. Children can say: “Hi! Let’s be friends!”. They can be friends for an hour or for a few months. Older people have just two or three friends. (while not incorrect, I'd say "just a few friends") They have had enough of this. ("this" is a little vague, I suggest writing "they had enough friends" to clarify, or just "they had enough") You can know a person for a few years till (Again, not exactly wrong, but until sounds more natural in writing, unless you're referring to a time period ex. "I'll be here till 7 pm") you understand (understand sounds a bit too logical, like you've calculated your trust level :D I'd suggest "believe") that you can trust them. This person is really your friend, and most likely they will be your friends to old age. If you can say: “I was lucky. I have a good friend” you are a happy person.

In green are some corrections (mostly concerning plural vs. singular form, that may be your only mistake c:) In blue are more stylistic corrections - they're not wrong per se, but they don't sound very natural. I hope this helps, and best of luck! c:

Friendship.

What do you think about friendship? How many friends do you have? In our childhood we think that we have a lot of friends. Children can say: “Hi! Let’s be friend!”. They can be friend for an hour or for a few months. Older people have just two or three friends. They have enough of this. You can know a person for a few years till you understand that you can trust them. This person is really your friend, and most likely they will be your friends to old age. If you can say: “I was lucky. I have a good friend” you are a happy person.

Good job and good message

It is very true especially with social media and people spending too much time online.

Actually I made a funny short video about it. I hope you enjoy it.

https://www.youtube.com/watch?v=6zUzal-CAII&feature=youtu.be