I ride a '00 CR250 and my two sons (12 & 14) ride '01 CR80's at New Jersey Off Road Vehicle Park in Chatsworth, NJ. They have 4 tracks (quad only, beginner mx, intermediate mx, advanced mx). There are also a number of directional trails in the park.
You can get a yearly membership or pay by the day.
Their web site is here => http://www.njorvp.org
From the main menu page under "Park Info." click on "Memberships" for prices and park information.