E-ticket does not actually mean you need a card. Many travel agencies for example just print a paper that you bring to check-in, works perfectly well.
The only thing the check-in agent actually needs is the booking reference to be able to retrieve the reservation, if that is done by swiping a card or by manually entering the booking reference makes little difference.
When you travel on an e-ticket you will still get a receipt at check-in to show at the US immigration (proving you have a return ticket).

I think you can also store e-tickets on Swedish bankcards (Maestro), and AFAIK you can get a Maestro card at the age of 15 as long as your parents agree and sign the paperwork.

What I think Tommy777 meant regarding the lounge is that gold cards and gold lounges don't exist just so that people can show off to others and feel important.
The lounges are there as a gratitude to loyal customers who have done a fair amount of flying with SAS and spent lots of money with the company.