This isn't something that can be anwsered with a simple number. Cynicism about annything is more a part of personality than anything else. It is beleived to develop as people age, and cynicism about a particular thing would probably begin as they start to lose something. If we base beauty as looking youthful then we could say late twenties to early thirties (so around 28-35) as that's when signs of aging begin to show.
