What does i am under retirement age for this plan mean

This means that you are receiving pension benefits earlier than you would normally qualify for them because of a disability.

For example, if the earliest you could have retired under this plan was ager 55, but a disability allowed you to "retire" at age 50 then you would be "under the normal retirement age for the plan until you turned 55." 

If you see this question on a state return, it may be for a state which does not tax pensions, if you are at the "normal" retirement age under that pension plan's rules.

That is, if you took early retirement so that you are currently under the normal retirement age, then the pension is taxable in that state (Pennsylvania comes to mind as an example), until you achieve the age for normal retirement under the pension plan's rules. Once you reach that age, your pension is no longer taxable (although you normally have to indicate this in the state return).

But since we don't know the context in which you saw this question, it's hard to know which answer applies to you.