You're the only one that's mentioned party members, they're usually oldies in all parties."The average age of full members has actually nudged up from just under 51 to just over 51"
I suppose that seems young to you.
I stated supporters, i.e. the young idealistic dreamers who vote for them.
Labour always grows in popularity while not in power, partly because many naturally dislike whoever's currently in power, but also partly because every election there's a new crop of young voters who don't have any adult memory of Labour being in power so haven't experienced the reality of what they actually deliver, so are more likely to believe whatever they promise. Even if labour don't win this time, they'll probably still hang around and vote for them the next time and beyond, mostly just until they get to experience the reality of them actually being in power, then they see the light.
They promise flowers, rainbows, sunshine and fairies. Easy to promise, impossible to deliver.