By and large the acceptable parameters are as follows for typical fish
Ammonia : 0ppm
Nitrite : 0ppm
Nitrate : 40ppm
Once these are above these ranges it's best to do a 50% water change. Typically, most fish hobbyists get away with a weekly 50% water change and don't bother to test the water, however it's much better to test more frequently.
For pH it's most important to keep it stable. It will typically fluctuate during the day, but as long as it's not a large change the fish will be fine. Stability is key with ph.
Ammonia will be detectable in a cycling tank, but as long as you keep it no higher than 0.25ppm your fish will be perfectly fine. Just do a 50% water change when it hits 0.5ppm.
These numbers change when keeping harder to manage fish such as discus or wild fish, so refer to species specific guidelines in those instances.
Lastly, age of water has nothing to do with it's ability to hold oxygen.