How does someone in the industry use the right icons and text together in order for age > 40 people to understand the functionality of the feature that they are clicking on. I have come up with a few small ideas where a button would have the text centred and the icons on top left corner small/medium sized but I am looking for more input or if there are any studies done for this?
Age is only one of the many aspects that determine the likelyhood that someone is technically literate. The common understanding that the older someone is, the less likely they are up-to-date with technology is true to some extend, but it is also a heavy oversimplification.
You need to look at your particular user-group. What kind of platforms/devices do they use? What conventions are they used to and prefer? If you don't know that, you can only rely on assumptions.
Generally speaking, using both an icon and a label works best. You don't have to gamble on users being familar with icons, as the meaning is next to it. Having both text and icons also helps as an anchor point when scanning, meaning users will see and identify the features quicker. The sizing of the icons should be on the larger side, as accessability remains important for any user group. Other than icon size, you should also pay attention to contrast, color and font legibility.
Over 40 is a very wide brief!
Eyesight issues will tend to kick in from mid 40s - and by late 50s some of the age group will have failed to keep up with the latest trends and will be starting to get stressed by anything "new".
Always almost, but even more so with an older user base, it's a good idea to not rely on icons to convey meaning at all and only use them as decoration.
In my experience, what a designer thinks is obvious, is commonly almost impossible for many users, especially older ones, to guess.
From an UX standpoint (if you definitely want/need icons), displaying them next to the text that further explains the action/whatever is the way to go. (I usually put them on the left side but that's personal preference)
Utilizing tooltips might seem like a great idea, but it often ends up being not obvious enough for users with a mouse, not viable for screen readers and impossible to use for those on touch devices.
You need to do more research. As someone in their mid 40s I have perfect vision. In fact my health and all cognitive ability is 100% much the same as many people over 40. Fitter, healthier, and more aligned with technology than most. So I struggle to see why you think your audience will not be able to learn new UI or UX just like anyone else. I say this as I am not unique, all my friends are tech savvy.