To be honest, I think it's a lot harder to tell someone's age just by looking at them (certainly when you get past your mid / late 20's anyway).
Once you get past around the age of 25, your lifestyle choices will determine far more what you look like than your actual biological age. For example, those who live a healthy lifestyle (i.e.. exercise regularly, eat decent / clean food etc.) are known to age a lot slower/better than those who don't exercise, drink heavily / regularly etc.).... also genetics and ethnicity may play a significant part to the ageing process. People like Liz Hurley, Hallie Berry etc. can easily pass for some 15-20 years younger than their chronological age.
Besides, IMHO, there comes a point when you just have to accept that someone is in no way a child but a fully grown adult and can do what they want, when they want... with whom they want. Personally, I would put that age at 25 years old (IMHO 23 is close enough to that age).
Besides Demi Moore and Ashton Kutcher were similar ages when they started dating... no on seemed to mind lol