It is about 8 year old. The last few years, it starts losing speed going uphill, now I have to push it like a skate board to climb even the smallest hill, only will ride around level and uphill in 1st, but in 2, 3,4 and 5th, works fine going downhill.

It cuts fine and when put in reverse, goes just fine uphill and downhill. Have changed the oil and the air filter, but that's it. Any suggestions?

It's a rear driven, basic model, smallest and most basic model they carry.

Thanks

So you are saying you can back up a hill just fine but can't drive forward up the same hill, right? Does the engine slowly loose RPMs when going up hill forward or does the engine remain a full throttle but the mower slows down?

May just need a good tuneup.
Compression test can't hurt and will eleminate as a possible culprit.
Glad all gears work going downhill, having to Push downhill could not be a good sign.

Sounds like you need to replace the drive disc.