I remember how relieved DH and I were when our cousin (who raised 7! Kids and now runs three care homes for deeply disturbed children) laughed about his own 14 year old who would look at the bathroom fittings with complete bewildered bafflement sufficient to convince any impartial onlooker that this child had never been cared for or shown how to wash. Every night!
In the army, new recruits are shown how to shower by an NCO who strips off, gets into the shower and demonstrates everything from ears to foreskin. Then no-one can claim "no-one showed me" and no-one is singled out.
Both my two went through a very grubby phase, as did my siblings. We're the first DGP's for two generations so we've no idea if scrubbing teenage GC Is part of the job description, but we'd have been grateful for support "yes we can tell you've not washed".." Etc.
People eating and drinking on the go