depends on a lot of things, if you take the time to train 10+ hours a week i'd say start racing in the spring as a CAT5 you won't be the worst! Then it all depends on how well you improve but it's not unrealistic to move up to CAT3 before you are 30 if you really work at it.
If you have a gift, 26 isn't too old to start racing and move up to pro level (not Europe but United States) by the time you're early 30's