When and with whom is it appropriate to to use คุณสวย?

I know it it acceptable with children, but is it acceptable to use with adults?

I'd say mainly adults.

Children more. (NaRak).

36 minutes ago, carlyai said:

I'd say mainly adults.

Here is the point. A neighbour likes to use this with my wife. I have not said anything yet, but as he is not a close friend, i think it is inappropriate.

1 hour ago, stubuzz said:

Here is the point. A neighbour likes to use this with my wife. I have not said anything yet, but as he is not a close friend, i think it is inappropriate.

I'd say your wife would say something to you or him if she thinks it's inappropriate.

Tricky one. :)

Did you mean คุณสวย / khun suay (you are beautiful) or คน สวย / khon suay (beautiful person)?

I'd say the former is used with ones partner or when flirting, and the latter is a common compliment which can be used with people familiar, not so familiar and sometimes jokingly.