Quote:
"You can use your Railcard on all franchised train operating companies that provide services within England, Scotland and Wales.
Railcards do not give discounts on Eurostar services, Charter services, and Heritage Railways unless stated otherwise."
Source:
http://www.senior-railcard.co.uk/help/faqs/validity/
Eurostar's Paris Senior Fare (which has nothing to do with the Senior Railcard) is available only on selected dates and can be booked by following the instructions here:
http://www.eurostar.com/uk-en/calendar/senior-availability-calendar-travel-66-return
You should note however, that the discounted fare is only available for bookings from St Pancras (or Ebbsfleet/Ashford). If you're travelling from elsewhere in the UK, it's usually cheaper to book through ticketing (at the regular fares) via this link:
http://www.eurostar.com/uk-en
(Click in the 'From' field, then on United Kingdom, followed by the initial letter of your departure station and finally on the name of that station).
It's also worth noting that, as long as you book far enough in advance (and avoid peak-time trains), you can normally get a £69 return fare between St Pancras (or Eurostar's Kent stations) and Paris. (If you put yourself on their mailing list you can sometimes get a £59 return fare). The minimum 'Senior' discounted fare, of £66, isn't that much cheaper.