With a lot of time and effort! I cannot read traditional music because I'm dyslexic and reading 2 clefs at the same time is simply impossible for me. I cannot play by ear because I'm quite tone deaf. So not easy!
I follow a letter note system because of that, but it's more for basics, chords and rhythmics, not proper pieces. I can only play actual pieces by watching piano tutorial videos, and simply copying the position of fingers and speed. Monkey see, monkey do!
Some tutorials come with the key names, so it's much easier and faster to learn. Of course it took me a while to get used to the letter names as I'm Spanish; Do Re Mi. Why Do is called C instead of A is still an illogical mystery for me, but so many things are when one lives in a foreigner country...
I learn very small bites at the time, and then keep practising them and adding the new bites. It takes me ages just to know where to put my fingers. Then, I focus on speed and expression. Normally I wait to learn the whole piece before this, but if it's a long one then I might divide it in half or even quarters and start practicing the speed and expression for each part, mainly so that I can see that I'm actually improving and that pushes me to continue learning. It can take me a couple months to learn a 5-6 min piece, so you can imagine when we're talking long ones. I try to put at least one hour a day.
I also tend to half sing the notes as I learn them (really, you wouldn't like to hear me!) and I find that this helps me memorising the piece.
Of course my repertoire is still quite limited, but I have some modern and classic pieces that I really like and play decently and this keeps me putting up with the time and effort. The most difficult thing for me is finding video tutorials for pieces I like, as many people do not share their skills. But luckily there are quite a lot of video teachers and websites for people who cannot read music sheets. "Artificial music" such as Synthesia is also of great help.
There is simply no way somebody like me would be able to learn without YouTube and the Internet, so for me being in my 40's is not really the problem. I tried to learn when I was much younger and my memory was better, but then there were simply no systems for dyslexic people like me.
At the end of the day, each person is different, and what works for one doesn't for other, so one has to try a lot of different systems to find which one works better.