Hang on - I think you've misunderstood what those time spans are for.
It's usually 12 months under 35 / 6 months over 35 - this is to trigger an investigation into what's going wrong. Because you can't just run fertility tests on every member of the population who is impatient because they didn't get pregnant the first couple of months of trying, so there has to be some kind of criteria to meet first.
But that's not your situation, you already know that your DH has low sperm motility and morphology. So if I understand correctly, you should be able to access an appointment to discuss options (even if you are not advised to go straight to IVF). That's not what the 6/12 month wait is for.
But to answer your original question:
The reason you get longer to try when you're younger is that assuming totally normal fertility, you only have about a 20% chance of conception per cycle, so it's normal for it to take a bit of time/a few attempts to conceive - the average is about 3-4 months (4 months is when it tips over to more than 50% chance). After 6 months you're at 74% (roughly) chance of pregnancy, but 11 months is the 91st centile, which is probably just rounded up to 12 for ease. So for 1 in 4 couples, even though they aren't pregnant by the 6th month it doesn't mean that they WON'T get pregnant, it just means that they have to wait a little bit longer. There is some quite significant randomness - you know when you're playing a board game and you really really NEED to roll a 6 but sometimes it can seem to be ages and so many turns before you get one? And then some lucky bugger on the other side of the board keeps rolling 6s and they don't even need one. It's kind of exactly like that - 1/6 isn't far off the probability of conceiving and then not miscarrying.
At 5 months trying, assuming normal fertility there's only about a 67% chance you'll have conceived. So for 1 in every 3 couples it takes longer than 5 months. That's very common - you are likely just in that 1 in 3 (or 3 of 10) and 2 out of those 3-in-10 will have conceived naturally by 11 months.
If you're younger, you HAVE time to wait and you can afford to say OK let's just try a bit longer, it is quite likely that we've had a crap roll of the dice. (And even up to 12 months doesn't necessarily mean that something is wrong, but it does mean you've been trying for longer than most people have to try, so it's absolutely worth investigating.)
If you're older, you MIGHT not have as much time to wait, the referral, waiting time, process of doing all the tests on both partners, waiting for results, appointment to discuss results, then yet more trying with whatever method is suggested, first pregnancy might not be successful and may need to try again - this all takes time, and in the meantime, you're both getting older. So they kickstart the process after a shorter period of time, even though there's still a chance nothing is wrong, because you DON'T have loads of time to just wait around trying naturally, and also because of age there is an increased chance of issues anyway so the bar is lower to check.
Someone in their 20s or early 30s does have time on their side. I would ask about the possibility of an NHS appointment to discuss the results you've had (if you haven't had that already) but in the meantime, keep trying, because it's really not that unlikely that the odds will come up for you at some point. And if you have a condition where the result is that it's possible, but less likely, then actually one way to handle that is just to give it more time. This has the benefits of avoiding all the invasive, painful, and risky parts of IVF as well. My DH has a chromosome translocation, which basically causes genetic defects but these can occur at various stages of development depending on where the break in the chromosome is - for us it seems to cause it so early that I either miscarry or the embryo never implants, which IMO is the easiest version of this to cope with. (As we have been lucky not to have to cope with TFMR, later miscarriages, PGD IVF, difficult decisions.) It took us about 15 months to conceive DS2 and then after he was born we decided not to use any contraception and DS3 came along exactly three years later. (DS1 has a different dad.) Some people use IVF with our condition but we felt that we were lucky to be able to just wait and see.
And BTW, worrying or not isn't going to make your DH's sperm swim any differently, so worry away (or at least, don't beat yourself up/worry about worrying!!)
To go back to the stats/probability - I found this reassuring, it isn't for everyone -
If for example you had half the chances of a couple with totally normal fertility, then you still hit over 50% chance by month 7, 75% chance by month 14. And if your DH's sperm count is only borderline for investigation then you probably don't have half the chances, it's probably somewhere higher than this.
But essentially what I'm saying is that the fact you've been trying for 5 months with no results is not yet a reason to stress or automatically assume that it will never happen. It is likely that you just need more attempts. Think about playing that board game where you're really really desperate for a 6. It usually arrives in the end!
Lastly (totally unstatistical, unscientific advice) - try a cock ring. It's meant to make the ejaculation stronger so it shoots further. Could have been total coincidence, and apologies because this is TMI, but it worked for us with DS2.
Good luck!