Is there a bike age limit?

As long as you think it can match the competition of the other riders don't worry about it. It's fine

In simple terms - if you passed a CBT at 16 on a NON geared moped - you can only ride a NON geared moped for the 2 years your CBT lasts. (Unless you take another CBT after turning 17 on a geared 125cc bike, which will last 2 years from the pass date) If you then wanted to do a restricted liecence you can do this once you turn 17, and you would be able to ride your 600 CBR so long as it is resistricted to 33BHP, but you will only be resistricted to this for 2 years after your pass date - you will then be free to ride what you want. If you do a CBT at the age of 17-20 on a geared bike - you can then do the restricted liecence in which the same rules as above applied. The restricted liecence is taken on a 125cc geared bike. If you are 21+ you can do a direct access course, which means you need to do a CBT on a 125 geared bike (unless you already hold this liecence and it is still in date) You then do the full non-restricted liecence on (normally) a 500cc geared bike, and once you pass that you are free to ride what you want, with no restrictions. EDIT - also forgot to mention that both the restricted and non-restricted liecences require you to have passed the Motorcycle theory test, and they are the same for both the liecences. This is not required for any CBT.

Most bicycle racers are especially snobby when it comes to equipment. If your bike performs adequately FOR YOU, then just go for it. It would give you an extreme sense of accomplishment to destroy your opponents on "outdated tech."

All that said, what bike do you have? Sometimes people think any bike with a drop bar is automatically a "racing" bike. That, of course, couldn't be farther from the truth.

there are safety regulations for the racing bikes. Working breaks etc...as long as it complies with it then its fine

As long as it is safe, and wont harm you or other racers...

Must be road legal if riding on a road aswell...

If you can win races with it, that's what counts.