Thanks for the pix. This is assuming it is not a problem from too much water...
You can, and should, cut back a stalk or two of an aglaonema every once in a while. I would cut back the most bare, or the tallest and (or) anything that seems to be falling over. Just cut the stalk, about 2-3 inches above the soil level. You can root the cut stalk(s) in water or moist sand if you like propagating your plants.
I am not sure what that is I am seeing on the top soil. Is it a crusty buildup of some kind or you have pebbles on the top? Or maybe it is old moss? Anyway, if that is soil you should remove it, then if you can remove the plant and rootball from the pot without damaging the plant, add fresh soil to the bottom of the grow pot, (check the roots while you have the plant out and make sure they aren't rotting or anything) then replace plant in the pot and add soil around the sides and just a little on the top. That will give your plant some fresh nutrients.
Remove any yellow leaves and continue regular care. It really looks okay considering it is five years old. It is a good habit to prune back the plant a little every year or 2 just to keep some new growth at the base of the plant. You will end up with a fuller plant in the long run. If you let it grow taller and taller, it will lose the lower leaves and fall over eventually.
You can let it grow tall if you want, that is your decision.
The leaf loss may just be normal. If it continues, really check your watering as that is often the problem with houseplants. Checking the roots to make sure there is a lot of firm healthy roots will tell you alot. If the roots are minimal, you may be keeping too wet. It usually takes some time for problems with too frequent watering to show up. Unlike underwatering where you will see immediate wilting.