To decide if a deck needs replacing, you can look at the wear of the wood. Is it starting to splinter or crack? Are the steps sagging?
If you have young children, check the gaps between railings to make sure your kids will be safe on the deck.
The building code today for decks is stricter than it was years ago. Check that your deck is still securely fastened to your house.
If you are concerned about your deck, feel free to contact me.