I would tell a person of any age to leave a relationship that was unfulfilling.  That is no way to live.

Being single and not having sex is not bad at all compared to being in a relationship and not having sex. You don't feel constantly rejected and deprived, you feel excited about the possibility of having sex with someone new and seeing them naked for the first time.

The best moment to jump ship is now. Don't wait. Break up today.

I left at 53 and I wish I had left DECADES sooner.  I wish this every day.  I have been free for 12 years now.  I have enjoyed every minute of my freedom.

Here's the deal the way I see it.  The rest of your life is the rest of your life.  Maybe you will live to 100.  Maybe you will get hit by a truck checking the mail tomorrow.  Take back whatever you have left and do what you want with it.  Tomorrow is never guaranteed.  

I am now so old that my age appropriate dating pool is 60 to 70yo men.  Half of them have ED to one degree or another.  Then there are those just looking for a housekeeper.  Or a nurse.  Or an ATM/free ride.  So way less than half the men in my dating pool are interesting to me.  Is it hopeless?  No.  Just more difficult.

Does this make me wish I was still with my LLH?  Oh Hell no.  I am still glad every day that I left him.  I am still up for a challenge and I will deal with it just fine.  But life with my LLH....THAT WAS LIKE TRYING TO RAISE THE DEAD.  I have sk

> What is the last moment/year of life you would tell someone to jump ship

There is no specific age, moment, or time-limit. 

Rather, if you are consistently dissatisfied in your relationship, it's time to leave, whenever that may be.

This is true, but you obviously don't have to only date people who are your age. Anecdotal experience tells me that a man in his early thirties can still date women in their early twenties and up. Your pool is actually at its widest around that age.

OP asks a good question though, which seems to be at what age does the dating pool start to shrink for first time unmarried people.  Perhaps I misunderstand. 

If this is accurate, I'd think 30 is a close guess to when you'd see a noticeable decrease in single, never marrieds.  But this is anecdotal at best. 

The thing is, by 35 there are a lot of great people that come back into the dating pool via divorce.  More baggage to carry but perhaps better dating prospects than many younger unmarried people.  Older, wiser, likely know better what they want and need in a spouse.

You don’t need to guess, there is data on this:  

https://fivethirtyeight.com/features/when-will-everyone-i-know-be-married/

>So at what age does dating become harder and harder?

I'm in my 50s and have found dating much easier than it was when I was last on the market, in my 20s. I'm a woman though. I get the impression it tends to be more difficult for men my age, from talking to them. A lot of women are just done with men and relationships.

I think you bail at any age. If you only had one day left, how would spend it? I think not thinking about that question or thinking too much about that question keeps a lot of people here.

I'm 11 years in, and it just gets worse. Run!

Don't look at it that way.

Instead... we meet, get to know and begin to date folks to find out if we're compatible with them in many important ways.

Sadly, too damn many look past things and make excuses... say things like it will be better when and fill in the blank... when we move out of our parent's house, when we move in together, when we get a real job after school, when we're done with school and on and on.

Don't hope things will get better and waste time, years or decades even.

Don't say things are stressful and it will be better when he/she/we're not stressed.  That's bullshit, life is stressful and it usually get's more stressful (kids, mortgage, bills, sickness etc.).

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Far too many choose to go down with the ship when they didn't have to, they chose to.

I think once you make it to your mid 80s, the odds of finding a long term partner starts to decrease greatly.  That being said, many people don't start having great sex until their 60s and 70s, so anyone you meet after 80 is more likely to really know their way around "Magnificent Sex".

Your jumping ship because it’s sinking fast and not looking for the next opportunity,

There’s options in every age group.

I'd say after 40 is when everyone pretty much is less desirable.

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