You kind of making an assumption here I take it, unless you saw some identification with their age on it? Don't forget that some people age better than others and some people smoke (which is aging) and some people have hard lives with lots of stress which is also aging. You might even have some people that look younger than their real age. I've had been tell me I look early 30's up to mid 40s, so it's also about perspective and what you expect someone to look like at a certain age.
You can't really get annoyed with someone for not looking the age you think they should look and the poor person you have accused of lying by 10 years likely isn't 10 years older at all. I guess you could always get them some wrinkle cream as a gift if you go back instead of flowers
Happy St. Patrick's Day