Virtually all Beds baptisms and marriages pre 1812 have been extracted onto the IGI at
www.familysearch.org. If it's not there it's likely the person wasn't baptised for whatever reason, non-conformity being the commonest, or that he wasn't born in Beds
In 1841 he was aged 45 ie 45-49 as ages were rounded down to the nearest 5 below, and born in Beds. But both are often inaccurate in 1841. But the death date that you quote
(it was in the original wording of your message until you edited the message and changed the whole request!), 7 May 1846, was the burial date at Kempston of William aged 62. Burial ages are often inaccurate too!
The first baptism of a child of William and Elizabeth Garrett in Kempston was in 1813, with the next not until 1820. Different family?
In 1851 in Kempston his widow Elizabeth 66 b Chatteris Hunts (it's actually in Cambs) was living with son William 40 widower born Aspley, and son George 17 b Kempston, so it seems as though they moved from Aspley Guise to Kempston between 1811 and 1813.
William jnr was baptised at Aspley Guise on 24 Jan 1811, and Aspley Guise is only a mile from Woburn, where there's a member submission on the IGI, which needs checking as member submissions are very unreliable, of the birth of a William Garrett on 12 Aug 1788, son of William and Ann.
How do you know his father was also William?
(the modification to the original message has removed the statement that William's father was also William)I can't see an obvious marriage for William and Elizabeth c1810. I have the Chatteris transcript and there's no marriage there.
Woburn rates book site at
http://www.vmine.net/woburn/search_names.asp shows a William Garrett living at 8 Birchmore Green 1802-9
David