I have 1 year old domain that Im getting ready to list on Flippa - but the Site itself was just loaded on the domain about 1 month ago. I know its good to have older domains. So question is - what do I put for the Claimed Site Age?

Put the actual domain age or it flags warnings all over the listing.

Just be clear in your listing when the actual site was started and you'll be fine.

As far as i know you can edit it afterwards anyway.

I put a site up last week but put the domain age a day out and it flagged up with the old warning message so i edited the listing , changed the date and it was fine,

Pretty sure the domain registered date is what it refers to.