I didnt read all the posts...but...
THE RULE IS:
divide 2 add 7. any situation it works as the socially acceptable standard
for example...if you are 16, divide two = 8 add 7...15
if you are 18, divide two = 9 add 7...16
if you are 20, divide two = 10 add 7...17
if you are 30, divide two = 15 add 7...22
you get the idea...this is a fairly well known rule in Canada
so...im 21
21, divide two = 10.5 add 7...17.5 ~ 18
21, minus 7 = 14 multiply two...28
so my range...with this theory is: 18 to 28 (for 21)
works everytime