How far in advance do you HAVE to buy Frecciargento tickets?

If the reservations are made you don't have a ticket????

I don't get that one.

You can buy a ticket up until departure time if the train has availability. You will pay full price but you can.

Yes you can buy it at the FCO. Like the previous poster I don't understand what you mean that you already made reservations

We made reservations for a Frecciargento train FCO to Bologna. Can we
wait until the day of travel and buy at the FCO station, in case our
flight is delayed?

I'm confused too, Becky. If you have reservations, you already bought your tickets. Or are you asking if you can't use your already-purchased tickets because your flight is delayed, can you purchase NEW tickets at FCO (the answer is yes)?

If you have a reservation you have purchased a ticket. If you have a one of the various train passes, then it is possible to obtain a just a reservation in conjunction with your train pass. However, miss the train for any reason and the reservation is dead so you would need to purchase another reservation prior to boarding. If you don't, are caught, it is a very expensive fine paid on site immediately. So be very clear about what you are doing.

Given that you have made over 840 posts here, surely you are very aware of the ticket requirements for Italian trains. You may need to explain your comments.

Are you using a Eurail Pass? If you bought a reservation, then you don't need to buy a ticket, just board and take your seat. If you do not have a pass, then I think you tossed 10 EUR in the trash. The reservation is automatically included in the ticket purchase, so you will be paying for it again when you buy the ticket.

My husband wrote to Trenitalia because I travel with a wheelchair. They made a reservation for us with no payment required. Guess we’ll just have to wait and see what happens at FCO.

Frank, even though I have many posts, we have only bought Trenitalia tickets the day of travel several years ago.

Becky, let me guess, you are talking about telling them in advance you are travelling with a wheelchair and need assistance getting on and off a train. That is not a reservation in the normal sense of the word.
This is described here: http://www.trenitalia.com/tcom-en/Purchase/Info-&-Contacts/Disabled-passengers
More info here: http://www.rfi.it/rfi-en/For-persons-with-disability/Services-assistance-in-stations

for international travel, requests for assistance must be submitted at least 48 hours prior to travel
► for national travel, requests for assistance: if via e-mail, it must be made at least 24 hours before

You do not have to pay for this service. From what you have said you have done everything you need to do.

A reservation is a pre-allocated seat number. This comes automatically when you buy a ticket ticket on trains that need reservations, and is not possible on trains that do not. You will still need a ticket for this train, for which you have to pay.

Thak you, Chris. This is what we did.