Well, it is taken into account.
The test in law is whether a reasonable person would feel you had a genuine reason to feel under threat.
A 6 foot tall 220 pound war veteran like myself would find it difficult to argue that I needed to kill an unarmed 14 year old mugger.
A 19 year old girl in the same situation could almost certainly argue that she felt she was at serious risk of being seriously injured (which would include being raped) or killed by the boy - and she would be more likely to succeed in that claim.
It is taken into account in these cases.
You may use the level of force you reasonably believed to be necessary when all the circumstances are taken into account. That includes issues of size, experience - all circumstances are relevant because they are what lead you to believe you were in danger.
"Your Honor, the deceased's rap sheet provides preponderance of evidence that he needed killin' and my client should be released and commended."