It depends on why you are buying the insurance. If you only expect to need the coverage for a few years -- until your mortgage is paid off or your child finishes college, for example -- a rising premium policy (one where the premiums increase as you get older) would be appropriate and would minimize your immediate outlay of money. You could then consider dropping the coverage when you no longer need it. Increasing premiums might also be appropriate if you expect to have much higher income in a few years and you want to minimize the cost in the first few years.

On the other hand, if you will need disability insurance for many years (say, until you retire) you should consider a level premium policy. Here you will be paying somewhat more in the early years to keep the premiums from rising as you get older.