According to Official Guinness Records,
The oldest best man is Daniel Wilson Clement (UK, b. 24 March 1913) who served as best man, aged 102 years and 85 days at the marriage of Susan Davey and Alexander Little (both UK) in Castle Douglas, Scotland, UK, on 17 June 2015. Alexander "Sandy" Little has known Daniel Clement his whole life. Daniel was neighbour and friend to Alexander's Mother and Grandmother.
For a complete list of 2015 records, please visit 2015 Guinness Records in United Kingdom.