To restore harmony between generations & eliminate the problem of mass drunken yobbery in parks.
Before 'challenge 21' etc. since the '50s teenagers went into pubs when they could 'get away with it' around 15 or 16. Because they wanted to be seen as 'adult' they behaved & drank in a sensible manner overall.
When things changed they started hanging around parks drinking copious amounts of cheap booze, causing trouble & getting ill.
Let them back into pubs (they're going to drink anyway) & at least they can follow a sensible (on the whole) template of behaviour around booze.
It'll give them something to do socially & at least they could go dancing etc., something taken for granted by teens in the 'dance hall days'
For 12-15 year olds there should be far more adequate provision in terms of youth activity centres with music, sport, parkour, etc.