Is my female dachshund too old to have her first litter at 8 years old?

Yes a breeding bytch is retired at 5years old the latest.
I can't believe you've kept her fertile for this long, the risks of cancer, pyrometra and other reproductive diseases drastically increased with age in an unaltered bytch.
If your dog is generally healthy you should consider having her spayed.

ABSOLUTELY YES .... far far far too old.

Too old. Reputable breeders retire their females around the age of 6

WAY TOO OLD!