Capping the benefits a household can claim to £20k means they can still claim up to around £28-£30k in real terms, taking into account income tax and National insurance.
Not really. Most state benefits are tax free including the ones that usually cause the Benefit Cap to apply (Housing Benefit and Child Tax Credit). The only major benefits that are taxable are State Pensions and Job Seekers Allowance (there are others but those will be the big ones). So when it says £20,000 (or £23,000 in London) it really does mean that as you won't be paying any tax normally due to not having sufficient income to be over the personal allowance.
That's the thing I don't get. An adult is someone over the age of 18, so why talk about over 21s or over 25s when the minimum wage or benefits are being discussed. I think it's one thing saying the maximum amount of benefits are only available to people who have paid x years' of NI but apart from that I don't get a multiple tier system for people over 18.
Agreed. That struck me as being odd but it seems increasingly common to restrict benefits for young people wherever possible so adding in another tier in minimum wage I guess is continuing that policy.