Don't put anything down to just getting old- there are so many things that can be avoided, mitigated, or improved, might need a doctor to be aware of them, or mightn't be due to aging at all.
If you have any concerns speak to your GP or consultants. If your GP is crap then change practices (you can keep the old one for your kids maybe? Or isn't that allowed?) or I suppose you could see another one at the practice. I would seriously consider changing anyway even if they change with you. Your DC would benefit from a mum who is as healthy as she can be.
@Waitinginmycar Have you had any new stressors in your life? How is your mental health? It can effect how we feel physically.
I had the same with anaemia at one point- major symptoms when iron was only barely out of range. I took the prescription strength ferrous sulphate tablets (200mg) which I got from the pharmacist. It does take a while to feel better. I don't know if I would've paid £900 for an infusion tho, especially if I wasn't sure of its safety. I would've asked a mainstream NHS GP if I should pay for that treatment, is it worth doing for me personally if I'm willing to spend the money on it, is it ok, etc etc. They are usually happy to give advice like that.
I've never had any problem with being prescribed iron tablets whenever my iron levels have fallen below the normal range.
@ThinWomansBrain This did happen to me to be fair, they can have slightly different opinions about what's an ok range. Or they said it wasn't severe enough to be treated with tablets and should be addressed through diet, when I have an eating disorder.
I suppose I should've pushed for them to give them a supplement- if I'd explained that I struggled to eat enough etc at the time, I would probably have been given it.
@Waitinginmycar Is there a reason you find it hard to eat more iron rich foods? If your BMI is the low end of normal, you can well afford to eat steak, pate, whatever- or whatever the veggie alternatives are if you're veggie and don't feel you can start eating meat for the purpose I suppose.
Folate I had a deficiency of at one point too, and was given a supplement. But I didn't really feel that one, it was just picked up in a routine blood test or something.
I felt like I had 'some sort of deficiency' a few years ago; sluggish and achey etc, and my GP's response was to test for vitamin D levels, which were low, and prescribe a vitamin D supplement which I take in the winter sometimes.
I suppose they mightn't test it unless someone has the right symptoms to warrant the test.
They mightn't prescribe it to someone without a deficiency as after all there is no clinical need as they're not lacking in it, and if they gave unnecessary stuff to someone they'd have to give it to everyone. 
But the NHS website says that an individuals we could consider taking a supplement in the autumn and winter www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/ and for certain people throughout the year.
@Waitinginmycar Sometimes we can get symptoms and it's just one of those things, or we go through a phase of feeling a bit rough etc. I had a dreadful loss of balance and vomiting thing for a day a few months ago, had to call an ambulance as I was on the ground, but it was just one of those things.
they said this is just the ageing process, this how it feels to get older. Is it? Is it feeling dead tired all the time and seeing your body gradually deteriorate? Feeling slightly panicked!
We are going to see ourselves get old (hopefully!) and we will get more health problems and stuff.
I developed a couple of minor things, one of which causes an occasional slight ache in one part of the body but doesn't need treating and is normal for my age. The other thing needs a half-hour procedure I'm going to ask to be knocked out for. 
Agree with you that it's kind of frightening, I'm a complete wimp lol and even when I had to have an MRI as a precaution, I got full of cramps in my head and neck and got them to let me out. 
I take medications for two long-term, non-age related conditions, which are well-controlled.
Oh and I'm monitoring my blood pressure at the moment- it plays up for some clinics but has been fine when recorded at home first thing. I know I need to do stuff to improve my diet and lifestyle, even though I'm not particularly overweight.
Just letting you know all this to say
a) Don't let doctors fob you off. Try treatments/approaches they recommend and if it doesn't work, go back.
b) Do lifestyle stuff
c) Get therapy if you really struggle- it is hard to deal with what life throws at us and getting older is anxiety-provoking. xx Get help for anything that's effecting you emotionally and it might well improve your physical wellbeing.