Hi Neil,
Details of the young visitor discount are on this page: https://tfl.gov.uk/fares/free-and-discounted-travel/11-15-zip-oyster-photocard?intcmp=55575#on-this-page-5.
It says you can get the discount added at any tube or London Overground station. Romford is served by London Overground so should be ok, but if not then take it to a tube station once you get to London. Although that first journey will be charged at the adult rate, adding the discount will ensure that that days travel is capped at the discounted rate. Any blue Oyster card can have the discount set.