Why do you ask?
For me personally there are maybe 4 criteria:
a) Is she legally old enough. If not, then forget it.
b) Is she emotionally old enough? Some girls know what they are doing when they are 16, others are still unsure or confused when they are 20.
c) Am I interested? I met 16-yo girls who I definitely want to see closer. With many others I am just not interested.
d) Does she want sex with me (for whatever reason)? If not, then not. Fine. And if she wants it and all the other criteria are met, then let's do it.