As #1 says, you can find all the information you need at http://www.seat61.com to get current prices use http://www.bahn.de or http://www.bahn.com both a the official site of German Railways. Tickets are sold up to 3 months ahead of travel dates. To get "seniors" fares the site just asks for your age, and applies the relevant discount

Seven different routes, none direct, highly variable fares.

https://www.seat61.com/Germany.htm#london-to-berlin-by-train

Most convenient options are (only one change):

the Eurostar to Amsterdam and change to the direct intercity to Berlin (really early start at 6.16 arrival at 1826).

The other is the a Eurostar to Brussels and change to the European Sleeper service to Berlin (London departure at 1504 arrival at 05.59).

There's an even earlier option departing London at 601 arriving in Berlin at 1801 but it involves getting from Gare du Nord to Paris Est. A short walk).

"To get "seniors" fares the site just asks for your age, and applies the relevant discount"

I know why you would think that, but - no, there is no discount unless you have a railcard. The bahn.de site will indeeed ask your age, but the fare will be exactly the same for seniors as it is for travellers below pension age.

There are some countries that give discounts to all seniors, so there are some journeys for which you do want to enter your age - but Amsterdam or Brussels to Berlin isn't one of them.

I need to correct my earlier post... it would appear that senior fares are available on some routes after all! But the difference is negigible - the ticket I just bought for my husband changed from EUR 50.99 to EUR 49.99 when I entered his age.