Most I've seen seem to be in decline at around 30 years or so. They aren't long-lived, but they outlast junk like Bradford Pears.

Depends on the rootstock they're grafted on. On standard rootstock, they could potentially last anywhere from 40-120 years(or more)
I recently passed my boyhood home and saw the big old ornamental crabs(Robinson? Royalty?) that were planted back in the early 1950s - they were BIG when I was a kid climbing them and eating the fruit by the gallon, back in the '60s. Still going strong.

Just curious - what kind of crab and what do they want to replace this wonderful tree WITH?

We just planted one (Don Wyman Crab) and are going to put in one more on the other side of the house (front left and right).

BTW - I searched the net and you can choose 20 to 70 years as the normal answer with some much longer

I've had one (don't know the cultivar) alongside my house for 27 years. My guess is that it's 40 years old. It's been in decline now for about seven years, mostly apparent by the sparser foliage. It's also developed a lean and increased root suckering, but flowering has continued reasonably strong.

Grinder 12000:

They want to replace the trees with smaller Crabapple trees. Two years ago, our neighbors, across the street accidentally killed the three matching Crabapples that were on their side of the street. When they replaced them, they got much smaller Crabapples. So now the 2 sides do not look balanced - our side has 15 yr. old Crabapples, and the ones on the other side are about 5 yr. old (much smaller). I think this is a silly reason to destroy my 3 beautiful trees.

That is one of the worst reasons ever to remove trees! Tell them flat out NO! Crabapples are not that long-lived compared to some trees, but one cannot just make up for 15+ years of growth. That time is gone forever - it will take 15 years to get back to that level of shade, etc.

If they really want matching trees on both sides, then they should have required your neighbors to buy equivalently sized (and more expensive) trees two years ago. Since they didn't seem to then, I fail to see why making you replace your more mature trees now is a good idea. Making your neighbors NOW replace their trees with larger ones is much more reasonable, to my mind, than making YOU replace your trees - you value yours and, however accidentally, your neighbors did kill theirs!

In any case, 10 years, even another 5-8 years, down the road, supposing your neighbors are watering well, there will be much less difference in appearance. Tell the HOA to be patient! Either that, or have larger trees put in across the road. I almost think I would be quite rude, but guess you should save that for when you can't stand it any more..... Stand firm, and know you have support!

AMEN Dibbit!

I just wanted to make sure I was not being unreasonable about keeping my trees. Some neighbors will NOT be happy.

If these are the neighbors who are pushing for symmetry, now, suggest that they can buy larger trees for the neighbors across the way. (Maybe they can use the three smaller trees in their own landscaping?) If they aren't willing to put their money where their mouth is, then politely suggest they be quiet. Or not so politely; do you want a long-term relationship with them and on what terms?

Hard to believe anybody could be so callous and stupid to dare suggest harming your trees. Idiots! Tell these potential murderers to go shove it and learn to use their brains, if they have any. Doubtful. Don't they know the climate has changed and it's difficult to get trees established? It takes work and water and care to get a tree to "take" and live! Monumental a$$holes.

Time is precious, irreplaceable. 15 years! Imbeciles! dangerous idiots.

IN exchange, Tell them to provide you with liftime service of Apples!

I have one that's 42 years old and still looking good. Two others are almost as old.