That will vary greatly depending on the maturity of the individuals involved.
A rule-of-thumb would be to first ensure that you both have the same intentions for the relationship (e.g. friends with benefits, summer fling, or long-term serious relationship, etc.).
Another rule-of-thumb is to not have sexual relations with anyone until after you’ve discussed sexually transmitted diseases, birth control (in heterosexual relationships) and the precautions that you each expect to take. Anyone who is not comfortable having that discussion should not be having sex with anyone but themselves.