I'm working on doing background research for one of my novels and I'm looking for roughly how old a male in the peasant class would've lived around 5th or 6th century Britain.
According to Wikipedia, it was 30 years at birth. However, the reason for such mortality age is due to infant mortality. If you managed to survive until the age of 21, you could expect to live until 45 or (depending on source) mid 60's.
The average life expectancy for a male child born in the UK between 1276 and 1300 was 31.3 years. In 1998, it is 76. However, by the time the 13th-Century boy had reached 20 he could hope to live to 45, and if he made it to 30 he had a good chance of making it into his fifties.
Health A millennium of health improvement Sunday, December 27, 1998 Published at 05:56 GMT (BBC Online Network | news.bbc.co.uk)
The 5th and 6th centuries were the very start of the Early Middle Ages, a period of population decline and significant disruption caused by the collapse of the Roman Empire and the so-called "migration period". In Britain this was characterised by the Anglo-Saxon settlement which is thought to have led to widespread population displacement and frequent wars and instability. This contrasts with the population expansion and plateau seen in the late 1200s which are referenced in the answer above.
We can speculate therefore that life expenctancy in this period could be lower than that cited, perhaps between 25 and 30.