As many have said, (including in topics from the past) age doesn't necessarily mean performance decreases at all... I think it would be true to say performance/results would decline if you were at your body's absolute peak of performance, or if the level of training were to remain the same. However I think with age comes a certain wisdom - vets (and upwards) often have a greater level of dedication and more idea how to get the best out of themselves while running, and we clearly have multiple examples of that on this forum alone!
I'd liken it to a ladder - the rungs represents your ability, the higher you are from the ground, the better your performances, now imagine that ladder being stood in quicksand (or slow sand if there is such a thing!), if you stayed on the same rung of the ladder (i.e. maintained the same level of training), the ladder would sink and your performances slowly worsen, yet with age comes your dedication and know-how to climb further up the ladder. Annoyingly though, should you ever be lucky enough to reach the top of the ladder, there's nowhere for you to go but down.
...At 23 though the Vets category is quite a way off for me (as is any chance of prize money!), but i'll keep at it

Mucho respect to all the vets still at it (and usually finishin well ahead of me too!)