From the DSA web site.
"If you want to practise your driving with an accompanying driver, the accompanying driver must be over the age of 21 and must have held (and still hold) a full UK licence for three years. "

My opinion, as an ex. ADI (BSM)

As a 21 year old with only 4 years experience and no training in driving instruction you are only likely to pass on bad habits and incorrect driving tecniques.

The best way for any new starter, is to go to an ADI have lessons and also have someone accompany them, to allow them to get experience on the road.

The accompanying driver should NOT teach new manouvres.