Transistors don’t age. They keep working, staying within spec, and one day, fail without any warning. When that happens, as often as not, several other parts burn out as well (typically resistors). In other words, they work fine until they don’t.
The secret to long life for a transistor is keeping the transistor die cool. That’s a function of heat-sink design (more is better) and keeping the component well-ventilated.
The parts that do gradually degrade with time are electrolytic capacitors and paper-and-oil capacitors (not generally seen in modern equipment). Mylar and polypropylene caps basically last forever, along with adequately rated resistors.
So if you want your transistor gear to last forever, replace all the electrolytics every ten to twenty years, and get a small fan to keep it as cool as possible. Everything else should be fine.
Now if the circuit board design is really bad, with hot resistors next to capacitors, and poor quality solder joints, yes, it can fail in a few years, and can be very tedious to repair if all the parts are jammed together.
