There's a fair bit of information online about Bob Jackson serial numbers, you should be able to narrow down date of birth pretty closely. At one time it was possible to email the factory for information on a particular bike but they don't seem too inclined to do that anymore.
Very nice bikes for the most part, a common complaint was slightly misaligned bottom brackets.
This is a very interesting read http://www.classicrendezvous.com/pub...tombicycle.pdf Chapter 5 is all about Bob Jackson or JRJ cycles as it was called.