Forgive me for butting in on the thread as I'm only just about to turn 39 but in cynicism I'm advance well beyond my 40's if that counts.
I've never stopped playing handhelds owning pretty much everything that came out since the original Gameboy. Even took a Vita backpacking and a 3DSXL to India when I worked out there for few months a couple of years ago.
I've never had a problem with anything visually until this year as I start to enter that zone where age plays it's part on eyesight. In fact, if anything my eyes have been too sharp as I've always been able to see and discern things at distances that make friends, colleagues and opticians ask 'how the hell can you see that'?
In the last few months I've started to get what I'm calling focus lock-in. If I'm focusing on something near then my focusing doesn't instantly snap to far when I look away from it, and vice-versa. I've had this a couple of times playing Switch, reading a book or editing photos on a computer screen that fills my vision and looking up. Then I've had it a few times either watching a movie and then the lights coming up and looking at things near me, or watching a movie at home and then picking up my phone. In the worst instance which was last month I was watching a play in a theatre and during intermission I couldn't get a focus on any of the words in the programme during intermission.
It worried me enough that I got some advice. As it turns out, there isn't anything wrong with my vision per se other than natural ageing. What's happening is that it's only occurring under specific circumstances - ie I maintain focus on only one thing for a large amount of time. Basically, if I was multi-screening or looking at an array of focal ranges I'd not even notice it, but because I'm one of those people who once their attention is on something I rarely look away or take a break I was straining myself.
To mitigate and improve things I have to remember to look at different things around the room. Closing my eyes for 1-2 minutes is also like a reset button as are plain old eyedrops which I've now starting keeping at home and at work. Taking that advice onboard I've had a much more comfortable experience in the last month.
Of course it's different strokes for different folks, but I wouldn't rule out playing portables any more that I'd rule out reading books and magazines. You may find that your eyes just need a little more care and a change of habits as I did, but the best thing would be to check with your optician.
As for portable gaming in general. I travel enough that over the last couple of years it has pretty much been my primary way to play. With the release of the Switch it's actually become my main console thanks to the being a hybrid and I now don't have to fear starting a long game and not being able to play it regularly.