thirtysomething I'm sorry you are feeling so down about it.
It's a shame you decided to withdraw your application. I have to say based on your previous threads, you didn't say anything to make me think that was necessary at all.
I'm not sure why you think your decision was right to withdraw because they offered the job to someone else? Of course they did, you left them with no other option. You also don't say anything that leads me to believe that his gender, age or childfree status is relevant at all - I'm not sure where you are getting that impression from. I know sex discrimination does still happen but the kind of employer who gives the job to a young man because he is young and a man is very rare these days, even more so when there is someone in situ doing the job perfectly well already. As I say, nothing you've said indicates that your employer is like that. You say they have 'replaced you with a younger model', but you withdrew from the application process, so they needed to replace you and he was the best candidate left available.
How has it 'become clear' that they would have gone for him anyway? If you are similar on paper but you are more experienced at the job and they have always been very happy with your work, why would you think they would have gone for him anyway if you hadn't withdrawn your application?
I'm not sure why them asking after your health is relevant either tbh.
I don't want to seem critical, I feel genuinely sorry you are so upset about this, but unless there is an awful lot you are not telling us, I think you are being very unfair to your employer to be so disproportionately bitter about this situation, and unfair to your colleague to think the reason he got the job was because he is a young man.
Is it remotely possible that deep down you were scared of the process and scared of being appointed to do the job on a more formal basis, and have latched on to things that have been said and generated them into reasons to justify your decision to withdraw, and things outside your control that you can then blame? If in your mind they were never going to give you the job because they wanted a young man, then that's nothing you could do anything about, they are awful and you don't need to take any responsibility for the insecurity you were feeling.
I might be completely wrong, obviously, I don't know you or much about the situation. It's just a thought though, and might be worth exploring with yourself at a later date.
I hope you manage to move on, find something you want to do, can 'let go' a bit and start to feel more positive.