I got my first nice custom steel race bike in 1980. It wasn't long before somebody (who wanted to sell me a new bike) told me that it should be replaced after 5 years. After 16 years of racing, I did replace it but it wasn't done. It stayed in the lineup for many years after that. It isn't currently built up but may be back in service some day.
It's probably best to decide for yourself when you need to replace any of your possesions. Don't let a salesman make a decision like that for you.