The current ADA standards dictate the height of an ada handrail must be a minimum of 34 inches and a maximum of 38 inches from the walking surface to the top of your grab rail.
The height of your handrail must also remain consistent above walking surfaces, stair nosings, and ramps. In other words, even if your railing covers multiple surfaces (i.e. ramp, stairs, landing) your railing cannot be 34 inches in one section and 38 inches in another.
A special note is made in the ADA standards about the height of handrail that will primarily be used by children. In this scenario, a mid-rail with a height of 28 inches should be added below the 34-38 inch height grab rail.