Most PPE (including climbing & caving gear) is considered life expired after 10 years in ideal storage conditions, so out in the field you'd be looking at considerably less. A 30 year old hard hat is now certainly just a souvenir!
Mine was issued to me 44 years ago. It is certainly old hat.
The last time I wore it for protection was probably 20 years ago, after very heavy winds, when I went outside to inspect my roof. I wore it because I could see bits of tile around the patio, and was puzzled as I had just paid money to have my ridge and hip tiles re-bedded. I figured more could come down, and that the hat was better than nothing.
It turned out my roof was fine, and the tiles were from my neighbour's roof, which was subsequently repaired by the insurers.