Questions about doe fertility -- also anyone in W. Montana with good stock for trade?

#1:  For how many years can a female rabbit continue to breed?  The internet is broken these days and I can't find any useful information anywhere anymore.  I'll keep that rant short.  But I thought maybe someone here has some knowledge or experience with this.  My oldest does are 3-4 years old and it's been 6 months since they've given me kits, so I've been wondering.

#2:  I need to mx up my bunny gene pool. I just brought in a few bunnies from Idaho and looking for even more diversity.  I raise flemish/NZ/Cali mixes with a focus on disposition, size/condition, and colorations.  I get lots of oranges, blues, greys, and some combinations thereof.  Obviously lots of 'cottonail' and white as well.  They have fantastic fur and the adults usually top out 7-9lbs.  I wouldn't mind getting some broken patterns in the mix!  Anyway.  I enjoy trading, so if anyone is in the vague Missoula area we can meet up sometime and do an exchange!  I can trade rabbits, chickens, seeds, bacon, and a wide variety of other permie things.  Yes, I said bacon.

The majority of my does tend to start slowing down around 3-4 years, and typically retire from 4-5 years. I can typically tell when they begin to miss breedings more often and when they do take the litter size decreases.

Since the broken gene is considered dominant, you'll need to find a broken to produce brokens (2 solids can't produce a broken, even if their parents were broken). One option is to find a genetic charlie, which is an animal with 2 copies of the broken gene. They usually are lightly marked, with 10% color or less. Since charlies don't carry a gene for solid coat pattern, they can only pass down the broken gene to their offspring, meaning all of their offspring will be broken patterned. 🙂

There is a doe here who is now 4 years, 4 months old.  She has had three litters this year.  Her first one was 4, then 6, then 3 and then an unsuccessful breeding.

Hmm, the first and last litters were with a young buck under a year old.  The larger litter was with a 3 year old buck.

There could be reasons other than the age of the does for litters not to show up.  Bucks can become temporarily sterile in hot conditions, which I've heard can be 83 degrees Fahrenheit and above.  Check that number, though, I could be remembering it wrong.

Several years ago, we had zero litters after dozens of matings.  Turned out a baby orphan bunny we had fostered had been an orphan most likely because it's mother had died of of Vent Disease (bunny syphilis).  A year or two after that, the syphilis had gotten spread throughout the herd and no litters were appearing.  It took five weeks of each bunny getting a shot of penicillin to cure the herd.  

If it's just the older does who aren't having litters, then it probably isn't something systemic to the herd such as over heating or disease, though.  If you're still getting litters from younger does, then it could be as you suspect - the age of the older ones.