Some oddities of the law on age: So you thought you reached age 21 on your 21st birthday? – Wilberforce Chambers
‘Well, yes you probably did as a legal matter reach (or attain) age 21 at the start of your 21st birthday – ie at midnight at the start of that day (even if you had been born later in the day). But legally this has not always been the case in England and Wales.’
Wilberforce Chambers, 7th January 2020
Source: www.wilberforce.co.uk