I second the AMS suggestion. The Netherlands provides quick and easy access to mainland Europe.
As mentioned above, LHR is also good and based in London will allow access to the LCC (EasyJet, Ryanair). For this reason, I would disagree with the suggestion of DUB as Ryanair is connected from STN.
The lads may want to consider FRA, MUC or TXL (any Berlin airport). Deutschebahn has an extensive rail network that connects with other European countries including overnight series (much as the NL does). Germany has many youth hostels, many very nice.
Being post high school they will find their feet very quickly. Part of the fun and adventure is discovering how to do things alone. Ah memories
H