Replaced all my thruhulls in 2012 just after I purchased the boat. I had 3 of the original plastic one's that were cracked. Not good!
I went all SS, since I trailer, but it is worth replacing with either bronzed or SS! Then don't have to worry so much with age down the road with plastic again and sun damage. I noticed the plastic one's appeared to be sun damaged or maybe just age, but they were brittle and crumbly on the top layer.