I started into MX this sping at 31 years old racing a 2000 400ex . all i had done to the old girl at the time was jets air filter pipe rev box. everything was stock. all i added after i started racing and this was about mid season was extended rear hubs from RM. I did pretty good for my first year on the old girl and really inproved my skills and i did win the MX and SX championship in the 30+vet class at the local track here Miles Mountain, it was because i made all the races and had the points. But after saying all that it was still not in the same league as a 450.
i raced C-class the last race and had my hands full trying to stay with the bikes not the riders. they are just to fast in the staights.
I just bought a yfz450 a month ago and even though it is giving me fits allready it is still a better faster bike. I hope this helps, i raced my EX because its what i had at the time and i didnt know if i would enjoy racing, now i cant wait for next season. LOL!!!!!