I'm with you goingbackwards - it does no harm to dream when a girl's mind turns to trivial things. But at my age I'm not sure I could cope with it.
Bev if you are between 40 and 50 then I think a 15 year age gap might just work fo a while - but I think a brother's friend or flatmate is a definite no no. Too many problems if it doesn't work whatever the age gap.