Surely they have to be looking for work, going to appointments, showing they are applying for jobs though and are under constant surveillance for it, money being reduced or taken back if they earn anything and so on. It's not a case of doing nothing and the money gets paid into their bank accounts.
Though I only learned recently that you could claim UC while living with parents. I remember being laughed at by the job centre 30 years ago when I asked if I could claim when I left university and didn't yet have a job.