Everything is about interpretation, when I was younger even about 15 people at school used to ask me to buy them drink or cigs as I looked older, just like I was called fat despite being at ideal weight for bmi or actually underweight because I am broad built, when I got to 25 and also gained weight people often think I am about 24 even now I have reached 30 but at same time its if someone looks upon you positve or negative i.e if someone was indifferent they will give you a younger age, if they took a dislike to you it would be the upper range of their guess i.e if you looked 25-30 they would say you looked 30, if they liked you they may think 25 i.e lower end.
Women are often a harder guess these days due to the modern type of make up, the amount of women I have seen make up free that look their age or even a little older due to things like duller skin, spots, crows feet(even women aged about 20) that when have make up on you cannot see any flaws so assume they are younger.