Never been out with a younger guy but i've only ever gone out with people i was at school with or my ex, who was only a year older
To be honest I know very few younger men who are mature enough for me to want to - and besides before uni i'd rather things were all teh same age. Saves that one leaving one staying scenario.
My best friend Tom had 4 long term girlfriends (1 year, 18 months, 6 months, 1 year) who were all 1 year older than him... but then he's pretty mature