Something to understand in regards to human eyesight and age - it tends to get worse with age because the lenses of the eyes get more rigid and less flexible with age.
Because of this, eye conditions like astigmatism (double vision, issues with focusing) are more common as people get older.
What this means for the user experience is that while younger people may enjoy nice small icons that are subtle, older people may be making themselves squint to try and force their not-so-flexible lenses to see the user interface in the same way as the young people do.
That said, here are some potential problem areas:
SIZE - icon sizes need to be increased
If your audience is an older crowd, please take size into greater consideration. This has to do with the whole squinting business I mentioned. You want your interface to be easy on the eyes, not something that's making people lean forward and go, "Wait, what's what?"
A larger interface with bigger icons will also help address the whole, "I can't find anything on the navigation tree!" problem as well.
COLOR CONTRAST AND PATTERNS - contrast needs to be increased and patterning also needs to be consistent
Compare your selected pencil tool icon to the folder icon and the house icon or any other icon that has a darker fill color.
Notice how they are very similar in terms of color? Also, notice the subtlety of the color difference between enabled and disabled (I assume the clipboard is disabled)/very lightly colored icons (the ones towards the right half).
This is confusing because even though the pencil tool is selected with a darker square color-filled box, it is still sharing a very similar color to other non-selected icons like the house icon and folder icon. Similarly, the subtlety of the color difference between enabled and disabled is too subtle.
Additionally, you have a collection of icons whose coloring patterning/schemes/styles aren't all uniform (which isn't necessarily a bad thing or something you can address, but I am just saying this factually) as some are outline-only... like the i-icon and some are filled in with the darker color like the folder and house and camera icons.
Because of the lack of uniformity in coloring -scheme/style- (some filled in some outlined), it adds a sense of natural variation to the user interface which potentially introduces a place for confusion to find root. Because if someone is looking for a selected icon and they are looking for a filled icon... they could - at first glance - see something like the folder icon and think it's the one selected when it isn't because visually, there ARE already 'filled' icons.
For these reasons, I feel that increasing the color contrast and - if possible - improving on keeping the same pattern/scheme/style in coloring the icons (all outline-only or all fill-only or a combination of both, or, if you want to keep the variation, make sure there is enough variation between the non-selected icons and selected icons) would be helpful.
A specific way to perhaps highlight the fact that the selected icon is selected is to make its background fill color for the square be a unique color to the interface. Additionally, add a 1px border using a slightly darker (and again unique) version of the fill color. Something like this would make the selected icon specifically stick out more but not like a sore thumb.
To sum this section up, better contrast in colors (in terms of individual icons and in terms of interface-wide icons) and better uniformity across icons of similar states (interface-wide icons, icons re: selected, disabled, etc) will help to make the visual experience easier on the eyes. This in turn will also help improve the ease of which people can find the icons they need.