There aren't really any youth only tournaments, bar the once-a-year inter-Uni event up at Witton Castle in Derbyshire (which only started this year!). All tourneys are teams of any legal age (usually say over 14 only) and many teams have very mixed age groupings, e.g. 6E Chicks ages range from 17 to mid-40s & all stages in between.
Regular tourneys near Hampsire I suppose would be at Sparklie's site in Maidenhead, Berkshire & at Temple in Surrey, then you have to start looking at the South West league, Dragon's Lair in Essex or even Kidderminster (though if anyone knows any more, please tell).