What can I do to rectify army records as regards my grandfathers age in ww2. He put his age down, being too old to join and the age he claimed would have made him about 7 when my uncle was born.
Many men either raised or lowered their age in order to qualify to serve their country in both world wars, and original documents cannot be altered under any circumstances. An index entry, as in the catalogue on this website, is simply a precis of the information exactly as provided in those original documents. If you are compiling your family history you should just make a notation about the discrepancy in your grandfather's age when he enlisted.
I confirm the response that Debra has given you - under the Archives Act 1983 we are not permitted to alter the original service records of your grandfather. It was not uncommon for men who wished to serve to provide either misleading or incorrect information about dates of birth, health history etc if they thought the correct information would prevent them from joining up.