Ebay won't let me send offers on an older item from December, but doesn't say why.

[ebooksdiva]

Probably because the buyer has opted out of offers. 

[buyselljack2016]

Couple of possibilities

Some watchers  opt out of getting offers. At least one watcher would have to be opted in.

If offers have been sent to those watching an offer can not be sent again to the same watchers.

[wastingtime101]

Watchers could be old. eBay lets sellers send offers to recent watchers.

Here's some official info from the help page:

• Not all listings are eligible. Eligibility is determined by a number of factors, including age of listing, listing format, and rules that manage the number of offers buyers receive
• Items must be in the buyer's cart for at least 2 days to send an offer