I have always read that 10 years is the standard life expectancy. Something of concern should be that tyres age on the inside as well as the external edges. This can mean that they will look fine on the outside but be no longer safe.
Your tyres are 15 years old and to be safe I would change all 4.
I would agree with Trent's take on this.
Lack of use is worse than constant use when it applies to rubber (this is the same with pipes and other rubber parts on automobiles).
165 x 15 isn't a common size, so you will be limited to a small range of options; however, I belive Pirelli offer the Cinturato P4 as one choice.
