The BBC sent freedom of information requests to 45 UK police forces. Of the 31 comparable responses received, there were 9,581 recorded offences of a motorbike being stolen. Police only recorded an age for 1,812 suspects. Of these, 965, or 53%, were under the age of 18.
Source: https://www.bbc.com/news/articles/c5y91dg75ejo?at_medium=RSS&at_campaign=rss