This is not an original idea – I found it on a sailnet forum post, and the guy who posted it copied from someone else. Regardless, it’s dead simple, very effect, and it only costs about $30. Rather than writing a bunch of words that are obviated by the photos, I’ll just post the photos.
Made of .5″ plywood, it incorporates strap eyes for line guide/retainers, and a cam cleat to grip the line. Feet go in the windows. Slide the board up the line and stand up. Your helper on deck follows your backside up the spar with the halyard attached to the bo’sun’s chair. He cleats off, you sit and slide the ascender up again. Repeat until destination achieved. When descending, hold one side of the jaws open. This allows you to climb a standing line made fast and taut parallel to the mast.
Note that the fasteners’ ends are pointing toward the climber, and not the spar. This keeps the screw ends from scratching the paint or anodized finish on the mast.
I think most of us could get up the mast like this, regardless of age. Take your time, rest often. Take a bottle of water up there with you. There is no hurry, and things done deliberately without hurry are done more safely.