Usually people answer something like ''u cant newer be too old !!'' but for real.... u can.... I think, the line goes for something like 30 years.
Its only my opinion, but if u look old,people will look u like T_T or O_O and u cant do anything about it. But if u look young, thats whole different thing.
At the end, its hard to give a number to it, cuz age is only and only a number. Im soon 18 and people say I look like 15. Okay, I get it, but its only a number.
or maybe when people start getting wrinkles, or when you can tell by looking at them that they're old. Because some people age really well and still look young at 35. My mom is 40 but people are surprised when she says she has a 16 year old daughter XD
