I am a Male. When I was around 7 years old I was taken to my neighbor`s house`s basement and she (at the time was around 13 years old) started to touch me and asked me to do the same thing to her. She took my pants off and was experimenting whatever was in her mind on me. I don`t think if I was enjoying it because at the time I did not know anything about sexual enjoyments feeling. It was really strange, but NEW for me. I was wondering if this experience is a sexual abuse or just a natural thing?

That is sexual abuse yes. She abused your trust too. What she did was wrong and she knew that and knows it was wrong by taken you out of sight to do what she did she may have seen it as experimenting but she also knows its wrong kind of experimenting you should report it its never too late.

Flip it around. If it was a 13-year old boy touching a 7-year old girl, folks would have very strong feelings about that. Just because it's the reverse doesn't mean that it's any less traumatic. Seven years old is seven years old, and way too early for sexual touching whether it be male or female. The age difference is also significant. Two kids in kindergarten/first grade might be curious about their bodies, but when one is so much older than the other...

Perhaps this girl was being molested at the time (or in the past), and was acting out on that.

If it haunts you or you feel bad about it still, talk to a professional to see if you can resolve the issue.

Although inappropriate--I think it was natural curiosity on her part.
I'm in no way trying to minimize your experience--especially if it was traumatizing for you! I was sexually abused numerous times starting at age 8 (but by adults) so I can relate... but if you aren't freaked out about it--my gut response is that it was curiosity and experimentation (although inappropriate).
Like Phoenix said... If it is haunting you--maybe you can go talk with someone about it.
I wish you well.
True.

You were very young and didn't know any better, but she did. Some of her peers have probably had sex, she probably had gone through classes about it in school, and what about PG-13 movies? The average 13 year old knows enough about sex to know that this was inappropriate.

It is only natural for humans to desire sexual intimacy, and it is natural for 13 year olds to have a curiosity, but this does not make what she did right at all. Acting on that desire when you're under the age of 18 is illegal, and is wrong.