Just wondering....I know it's recommended that teats should be replaced every three months, how about the bottles? My son is 1 years old and we've been using the same bottles since birth, only replacing the teats every 3 months. The bottles look fine, just the teat holder and lid look a bit yellow after so many washes and sterilising treatments. I'm thinking that my son should still be on formula till at least 2 years old so wondering if it's necessary to replace bottles. Any advice? Thanks in advance!