Just adding to Dr. Allen's post, I consider myself a male alto, counter-tenor (for want of a better word), and my range has slowly slipped from singing above the treble clef in my 20s, to a top note of treble clef c or d (con belto) and a good alto voice down into F or G of the bass clef (without "changing gear") in my early 50s. I sing in cathedral choirs and have turned into a useful Purcell "verse anthems with ledger-lines" specialist.
My tenor voice goes up to middle c or so, so there is an overlap of four or five notes. When tired (Sunday mornings), I can get a B below bass clef.

In answer to the question, the main criterion is to be female.