I had a flight with Avia Special at Barton as a Birthday present - it got me back into flying! The chap getting in after me looked about 70, so there wouldn't appear to be an age restriction as such. The route into the front seat requires a degeee of suppleness, but provided your pop can lift his legs as high as his waist, he should be able to clamber in.
He will get a blast out of it, although the airspace around Sywell is much more conducive to enjoyable flying. Going direct to Avia Special should save something over the 'Birthday Experiences' sold by Boots/WHSmiths.