The software running on the smartphone in your pocket has a bunch of options to help improve readability, and whether you have a serious problem with your eyesight or you just want to spend less time squinting, they can make a big difference to usability. Here's how to get text looking bigger and sharper on the latest versions of iOS and Android.
You'll find the accessibility options in iOS 8.1 by opening Settings and tapping General and Accessibility. There's a Larger Text option here that you can toggle on or off as well as a Bold Text feature that requires a device restart. Tap through on Larger Text and you can set your preferred text size, which will then be applied system-wide.
The new setting will affect all of Apple's apps, like Settings itself, Mail, Messages, the App Store and so on. Various third-party apps may not follow this lead, though, so you'll have to check within the settings for each individual app if they don't fully support iOS' accessibility features (most do). Safari doesn't automatically increase the size of text on websites, by the way, but if you activate the Reading Mode feature (tap the icon to the left of the URL field) then the accessibility settings are applied.
Other tricks you can try to improve the legibility of text on your device are to make use of the Zoom and Increase Contrast options on the accessibility menu. There's also the Display Zoom feature, which you can find in the Display & Brightness section of the Settings app. switch this on to make home screen icons and text larger.
Android doesn't have a slider, but it does have an on/off switch for bigger text. From the Settings app select Accessibility and then Large text to switch it on. As with iOS, you'll find this new setting is applied just about everywhere, from the lock screen to Gmail, though you may occasionally come across a third-party app that refuses to play along.
Elsewhere on the accessibility menu you'll find color inversion and high-contrast text options that may be of use for you, though both of these features are labelled as experimental in the latest Android build. High contrast text will occasionally add light or dark outlines to text on the home screens and in certain apps to make the words stand out more easily.
Finally, there's another option in the Android accessibility menu called Magnification gestures. Activate this and you can triple-tap on the screen in any app to enlarge the text or whatever is showing at the time; triple-tap again to go back to normal. Triple-tap and hold and you can pan around the screen without lifting your finger up from the display.