Backyard mint too grassy I just moved into a house with a large quantity of mint growing in the yard.

It is late in the season and a lot of the mint is going to seed.

I picked two packed cups of leaves and trimmed off all the stems.
I pulsed the leaves in the food processor with granulated sugar until it was a paste.
I then used the paste as the flavoring for homemade ice cream.

The ice cream texture was fantastic but the flavor wasn't. It was weirdly grassy and not very minty at all.
The kids wouldn't even eat it until I added a good quantity of mint extract to fix it.

I thought the mint leaves themselves were minty enough.

This is my first time using fresh mint this way.
Did I use the mint improperly or is mint only used very young?
While it is true that there are a variety of mints, I think your biggest challenge is that it is "late in the season."  I find that here (Philly, USA), in August, all of my herbs tend to develop a bitterness that is not there in spring and early summer.  While it may be the variety, I don't think it is the age of the plant, as my mint comes back each year as well.  It seems that the "soft" herbs are just past their prime now.  My rosemary and sage are fine, thyme is ok, but softer herbs like basil and lovage have a higher bitter component.  Try your mint again in the spring.

Another thought: Pulsing into a paste my also bring out flavor components that are not desirable.  What about steeping the leaves in the milk and cream (this would work if you are making a custard based ice cream and heating the cream), then straining them out?