Had to look it up, cause I wasn't sure of the exact definition:
The term "statutory rape" generally refers to sex between an adult and a sexually mature minor past the age of puberty. Sexual relations with a prepubescent child, generically called "child molestation", is typically treated as a more serious crime.
Statutory rape generally is any sex that occurrs when a person consents, but does not have the legal right to. Hence statutory. There isn't an age limit. No injury is inflicted, but there is a legal prohibition because consent is not recognized even if given. So, it could be sex with a minor, sex with someone who is intoxicated or under the influence of something, sex with someone who is mentally handicapped, things like that.
It is different from an act of rape, where an injury is inflicted.
The law treats them similarly, but they are not the same thing.
Under the age of 14 (typically) there is a different charge, something like "lewd and lascivious conduct with a child", which has broader scope (thus easier to prove) and typically has a harsher punishment. So the perp usually gets charged with that if the victim is under 14. Sometimes they get charged with both. If you look at what Philip Garrido was charged with for example, you will see both charges when she was under 14, but only the rape charge over 14.