That's the second time you've asked the question and we still don't really know what you mean!
Formal education in most states of USA starts at the age of 6 (although, just like in the UK, most children attend pre-school, kindergarten or similar). So their Grade 1 is equivalent to our Year 2.
As in the UK, if a child moves into a new town they will be allocated a place in a local school. So, if you emigrate to the USA, and your child is in Year 5 here, they'll go into Grade 4 in the USA. If your child is in Year 11, they'll go into Grade 10.
So, as long as they're not over 18, there is no maximum age for entering school in the USA.
Chris