Q: my old imac won't startup in recovery mode

What model/year is your iMac? You may need your original install disks to erase and reinstall.

If it’s running 10.6.8 or earlier, or a copy of Lion which originated from one of Apple’s installation thumbdrives, it won’t have a recovery partition, and a disk needs to be used to erase the drive.

(142407)

See above: you need a disk; you can get one from Apple with Snow leopard on it for $20. If you don't want to do that and are simply donating/recycling it, I'd suggest removing the hard disk and using a hammer on it before discarding it.

At newest, it’s a 2004 iMac, and it has a G3 or G4 CPU. You’d need its original disks or a retail 10.3.2 or 10.3.5 package to start it up for certain; the other compatible disks depend on its exact model.

(142408)

That’s an OS, not the computer’s age. Choose About this Mac from the Apple menu, click on More Info, find the processor type, and if possible, model identifier. A slot-loading iMac G3 can start up from a retail 10.2, 10.3, or 10.4 disk, not including the Mac OS X Server Universal Binary.

A Snow Leopard DVD won’t work in any case.

(142409)

Ok, so it's a PowerPC which means my suggestion won't work. You will need one of the disks Niel suggested or do as I suggested: take out the hard drive and destroy it.

You have a 2003 iMac G4. Buy a retail Mac OS X 10.3, 10.4, or 10.5 package from a source such as Amazon or eBay, or phone Apple and order replacement original disks.

(142412)