You don't have to split disability pension between earned income and pension income, but you can.
The reason for this option is to take advantage of tax breaks that require you have "earned income" (e.g. IRA, EITC,).
If you want to "split" the pension between disability and earned income, divide the total amount into two (2) separate 1099-Rs - one for payments before you turn 60 and one for the balance of the year - and enter each one on your return.