I have to agree. An immature 23 year old may in fact prefer dating younger people because they match their maturity age. On the other hand, a mature 23 year old may feel more comfortable with someone more their age and maturity level. Keep in mind though that everyone matures at different rates. Though it may be a rare occurrence, there are some very mature 18/19 year olds out there. The change of preference depends on you, just don't change your preference and then let it control you. You may miss your chance with someone really special who is a little younger (yet perhaps more mature).
Hope it helps.
Good Luck!
-Roxie-