Is there a senior one country pass?

The German rail pass is a special one, and indeed does not have a senior version. However, you can get a Twin pass if you order it from the German Railways: a twin pass is actually two passes for a slightly lower unitary price. 

https://www.bahn.com/en/offers/passes/german-rail-pass

Note that the German Rail Pass is only valid in Germany and a few international routes from Germany (interestingly for you also on ICE services between Brussels and Germany). It is not valid on the Eurostar or on other British trains, nor on the trains between Paris and Germany - so if you're intending to travel from the UK by train you'll either have to buy a Global Pass or separate tickets for the journeys to Brussels/Germany.

This twin pass will save us €61 or €76 depending whether we choose a 4 or 5 day pass! Thank you again, brilliant info. 
 

We will travel to London, then Eurostar to Brussels then into Germany with the German pass. Do you know if the tickets bought via bahn.com qualify us for a Eurostar passholder ticketb on the Eurostar?

Make sure you compare prices well. Deutsche Bahn has a special deal with Eurostar, that’s quite advantegeous. Tickets booked via DB, are often quite a bit cheaper than those booked via Eurostar.

From Aachen you can use your German Rail pass, (although it’s valid on the ICE from Brussels).

EDIT: Oops, Examples I gave compared two different trains.

Anyway, Eurostar tickets booked via DB to Aachen Köln or Frankfurt are usually a third cheaper. But you’ll need to add the price of the ticket for the ICE. Usually not that much, about 20 to 25 Eur per person.