Tyler Hamilton didn't start until his early 20's...and now he is a serious contender for the tour..and earns about a million a year.
Some top professionals started even later! Tony Rominger(now retired) didn't start until his late 20's...and he won a ton of races..and was Miguel Indurain's main rival for a few Tours de France and was one of the world's best time trialists and mountain climbers. Rominger even set the world's hour record before he retired. The UCI later disqualified his ride because he used an aerodynamic bike and position...still rominger has done the hour faster than any human ever has done it.
Then there was Andrei Tchmil.....who won several of the European classic races as he neared his 40th birthday.
There's a guy named Kent Bostick that still races at a high level domestically..and he's into his 50's. Kent made the U.S. Olympic team in 1996(atlanta) in the individual pursuit at the age of 43 by defeating the reigning world champion in the U.S. olympic trials. Kent is also a former 'masters' World Champion in several disciplines on the road and track.
If you want to go pro...I suggest you conquer things in this order....
First become great on the 'local' level
then concentrate on the 'regional' level.
By the time you are one of the better 'regional' rider you will begin to get some attention from regional level teams.
Once you are established on the 'regional' level..next move to the 'national' level. Here it gets tougher!
If you do well nationally..then people will start to take care of you...pay your expenses and cover your equipment with sponsors and such. Even still...don't expect to be making any serious money even at the national level.
If you have what it takes to thrive nationally...then the next step is international level competition which means that your butt will end up somewhere in Europe...like Tom Danielson is now doing with Fassa Bartolo after doing so well racing in the USA with team Saturn.
Here's my bottom line advice...BECOME SO GOOD THAT THEY CANNOT IGNORE YOU!!!!!! This will carry you far. Don't ever forget that advice. You have got to stand out.
Don't worry that you didn't start at age 16. Many cyclists that started that young burned out long before they reached their full potential...Alexi Grewal comes to mind. Even Lemond was fried mentally at age 32...and this is the age that Tyler Hamilton is beginning to excel. Lemond started cycling at a much younger age than did Tyler...so mentally tyler is still fresh. Usually when you see these guys no longer getting results when they hit their 30's..it's usually more to do with their minds than their physical abilities. It's hard to stay fresh after 10-15 years. Profesional cycling is so hard and demanding. The discipline required eventually wares ya down mentally.
You will not reach your peak anyway until you are 29 or later. Most cyclists are at their best from ages 29-32...which seem to be spot on regarding Lance Armstrong.