Dec 1, 2017 - 7:56 PM
Hello Petra and Welcome to MySwissAlps,
The answer to this question really depends on what your overall Swiss travel plans are. Taken in isolation buying a Swiss Half-Fare Card for CHF120 to save 50% off the Jungfraujoch fare does not make sense.
Leaving aside the savings for the 15-year old from having a Swiss Family Card with the Swiss Half-Fare Card you need to buy full-fare tickets to the value of CHF 240 for each adult just to reach the break even point. The Grindelwald - Jungfraujoch return ticket is CHF 190 so a Swiss Half-Fare Card would cost you CHF 120 to save CHF 95.
Without a any pass, children from age 6 to 15 pay half fare for the trip. Another way for your 15-year old to travel free is to buy a Junior Travelcard for CHF 30. See www.myswissalps.com/ju niortravelcard
If you let us know what your overall Swiss itinerary looks like we might be able to offer more targeted advice.