The simplest way to check whether you get a discount is to put your proposed travel details into the National Rail website:
http://www.nationalrail.co.uk/
Click on 'More options, railcard and passengers' to tell the system that you've got a Senior Railcard (or search normally, to get the regular fare, and then click on 'Passengers & Railcards').
Having just done that though, the normal single fare from Dover to Ashford (assuming that you mean the one in Kent, not Surrey!) is shown as £11.00 at peak times. Adding in a Senior Railcard sees the fare from the 0925 service onwards drop to £7.25 but the peak time fare remains the same.
Changing the details to include a return journey makes the off-peak fare (with a Senior Railcard) go up by just 10p, to £7.35. The peak return fare (irrespective of whether you've got a Senior Railcard or not) is also only marginally dearer than a single journey, at £11.90.
So the short answer to your question appears to be 'No'!