Gruess aus Deutschland-Ost now. There is N O automatic discount for age in either. It also depends partly on what type of train you use, what they call ¨Fern´ verkehr here (IC/ICE-though there will not be that many on this route, I think) or Nahverkehr=local/regional.
Also note-it lengthens the trip, but adds to various means and lowers total price: buy only to KEHL, the last german stop (hourly local train from Offenburg-which then runs on to Strasbourg/FR) and then use the (FRench) tram (streetcar) owned by Strasbourg city and no extra fare for going over the Rhine bridge=INTernational is needed. In that case you only pay local german fare till there.
Check site from ter.sncf for GRand-Est (the local area, includes former Alsace/Elzass) for possible other discounts or if its soon holiday-offers. There are also still daily tickets for train in either North/South-or both Alsace departments.