When to re-string ?
When you get a new racquet they usually come factory strung with a poor quality synthetic string this is ok for alluminium racquets but you need a good polyfibre if you are past this level.
If your racquet strings have lost tension over time you may notice but you can ask us if unsure.
If you have some pain in your arm you may need a softer string strung at a lower tension.
Juniors to teenage are better off with nylon it's softer on the arm and helps prevent injury. This may vary depending on the child's level.
With 30 years of stringing experience you can be sure to get the right advice!