How old should a child be before they are allowed to use the internet?

I think in current time 13 to 14 years is good to handle from good and bad things of internet.

I think 7 is ok if you help and control them =)

I think any age is ok. It just matters how you control their internet usage. A 3 year old watching teletubbies on youtube kids is technically using the internet. I think the biggest danger is interactions with other people rather than content. So it also depends on how you've taught your children about stranger danger and how to deal with it online.