"c_shah" writes:

I recently purchased about 15 year old house, I have Sears's
craftsman 1/2 HP automatic garage door opener Model# 139.53513. Anyone
idea how old this garage door opener would be? It suddenly stop
working and when I opened the unit it looks like plastic gears are worn
out. Is that something I would be able to fix it myself. I do not have
owner's manual.

That unit is almost new!

I just replaced one it must have been 20 years old or more.
At about 5 years the electronic circuit board went,
I replaced the board.
At about 15 years the plastic gear went,
I replaced the gear.
Just a few weeks ago the motor gave up.
I bought a new one, replaced only the
box with light and the wall switch and installed
the new electric eyes. The rest I'm saving for parts.

Do you think it is a good idea to buy new garage door opener? any
recommendation on brands? Should I buy chain or belt drive?

It's a lot easier to replace the gear than to put up
a new unit.

Stay with what you have.