A number of studies have been done on this subject. Based on reading a few summaries, there seems to be a consensus that happiness levels are high in youth, then decline with age and reach a low point in the 40-50 age bracket. One even specified the age of 47 as the unhappiest age for women, and of course it's a key demographic for male suicides.
Surprisingly, at least one study described the
septuagenarian bracket as the happiest of all. It seems to be a sweet spot in terms of not taking life seriously, being 'over the hill', enjoying the fruits of retirement but not yet being crippled by the various agonies of end-of-life health issues. I have known many people like this: stable marriage, a carefree lifestyle, abundant finances and no need to care about even the most serious issues facing the planet. They do laugh a lot.
However, reading between the lines, I feel like this definition of happiness seems to correlate with privileges and mental health more than age.
Personally, my late teenage era was
almost the most horrible time of my life, except it barely got better with age aside from escaping the daily torment of a sadistic family in favour of isolation and working crappy jobs. At 40+, it has plunged into pure blackness as I face the prospect of ageing having only experienced watching other people live life. I look forward and project being too poor to retire properly and rotting in isolation with ever-declining hopes of belonging anywhere. Also, as a sensitive person, I can't help but deeply feel the tragedies of the harm people do to each other and the ecosystem on a daily basis and don't wish to be a part of it.
To avoid being a total downer, I will say that there are many people for whom ageing is a graceful process. This may be due to a high-quality family, a supportive community, a rich engagement with passionate hobbies, a humorous/positive outlook, or even due to some non-material happiness via a successful spiritual path that defies a conventional explanation. It is unlikely any such person will reply here but a truly unbiased assessment of this topic should include their stories as well as the echo chambers of the unfortunate on this site.
But as much as I'd love to be a source of inspiration, my own story perfectly correlates with everything about ageing that you are dreading, and considering how hard I've tried to recover, I often wonder if I was just doomed from the start. All you can do is look at all of this very carefully, hopefully have a go at life and ultimately decide where you stand.