Are there Grand Masters racing sub-categories by age?

[Chris NewbyFraser]

I turn 5 years old at the end of this year (well, maybe I should add the other 60?) and would like to do a year of mtb marathon events on a competitive level. To those who know, are there Grand Masters A and B sub-groups based on age? A rider in his 66th year is at a bit of a disadvantage going up against a spritely 60yr old so I would prefer to be rated against my own ages(although I will still get hammered by the top gents).