> exploitation of children

https://www.google.com/search?q=exploitation
exploitation - the action or fact of treating someone unfairly in order to benefit from their work.

Age restrictions increase unfairness not decrease it.

> children protection laws: labor limits

Labor limits increase unfairness, and, therefore, increase exploitation.

> alcohol purchase

Alcohol [and other narcotics] consumption has negative consequences, especially in person's motivation system. These negative consequences [somewhat] justify restrictions.
What negative consequences does sex have?

> those laws are not ideal in all cases

"Not ideal" is a gross understatement.
Age discrimination laws are harmful most of the time.