It may no longer be effective if it is old. Sunscreen can actually expire.
Some bottles have expiration dates printed on them, some do not.
Sunscreen looses it potency after a year, especially if the bottle was exposed to sunlight in your beach bag, leaving you less protected from damaging UV rays.
Toss your old sunscreen and buy new. I like to write the date on the bottom of the bottle with a sharpie when I buy it. That way when I find a stray bottle I know if it should be tossed.
Have a sun safe day!