Is it wrong for a 29 year old woman to date a 21 year old man?

My cousin is 29 and she really likes this guy who is 21. They're both adults I know but I think my sister should be with a man that's HER age and stop going after guys that are 18-21. They're barely adults! My cousin is hitting 30 next year and she has been an adult for 11 almost 12 years! She has her stuff together but getting her masters degree. He is a nice guy and sounds mature but I think he's too young for her. I didn't say that to them but I'm just being honest and speaking my mind.