The big (number)-o phrase informal
An age that ends in a zero, especially as marked by a landmark birthday
My grandpa turned the big ten-o yesterday, and my family organized a great birthday party for him.
My ambition is to become rich before reaching the big three-o.
Used to indicate that the age at which a person's consent to sexual relations is legitimate.
Be nearly a particular age
Another individual is far older than the individual in question
Said when a person is in a romantic or sexual relationship with another younger person
Look older than a certain age
The origin of this phrase is not clear.