It's brilliant - whatever age. Over 60s get discounts. DH and I have travelled that way many times.
As other posters have said:
*you know this already- but get proper, serious travel insurance, plus the GHIC card: https://www.nhs.uk/using-the-nhs/healthcare-abroad/apply-for-a-free-uk-global-health-insurance-card-ghic/
*Seat reservations are ESSENTIAL on many journeys- I would say on most, for peace of mind. There is usually a small fee. The Interrail website has further details.
*if you want to see the passing countryside, avoid very high-speed (often premium priced) trains. Quite often, to avoid crosswinds, they travel between high concrete walls, so you don't see anything of the scenery. And the speed makes it all a blur. Local regional express trains are much slower but you see so much more from their windows.
*Get familiarised with how the 'composition' of trains is displayed. Many countries have information boards on station platforms, relating to each and every particular train. On some services, there is not much time to have to run with luggage from one end of a very long international train to another to find your seat. Best to be standing in approx the predicted position for your reservation carriage.
*Modern trains don't have much luggage space - it's either between the backs of seats, below the seats or in rather narrow overhead racks. So travel as light as possible. When younger for us that meant rucksacks, now we use small and light wheeled suitcases plus perhaps a 20-30litre rucksack (to hold on one's lap) for immediate essentials. As PP says, wheelie suitcases with rucksack-type straps can be good.
*Wherever you are going, a lightweight but really effective waterproof is very useful. As few shoes as possible - they are bulky and heavy. Toiletries can also be heavy; big Euro terminuses - eg Amsterdam, Koln - have good pharmacies for top-ups. (NB If at Koln station with children/and perhaps DH, don't forget to visit the fantastic free model railway installation.)
*I can see that sleeper trains might be very exciting for children, but really, we prefer a fairly basic booked-in-advance hotel or b-and-b. So much quieter and less cramped and better ventilated. And better breakfast, too.
*Most European railway station loos charge at least 50cents so be prepared with a supply of small change.
*Perhaps think about taking a universal adaptor plus a little travel kettle and (in our case) a small coffee filter cone and filter papers and some fresh ground coffee (or, if nor coffee fiends, teabags). If you can manage it, a couple of simple insulated mugs with lids are also very useful; the coffee sold on long distance Euro trains is streets better than anything offered by train companies in the UK, but not always available and always expensive.
Agree absolutely with comments re the seat 61 website - vast amounts of useful info there.