Maybe it’s not just aging eyes. The trend in web design is using gray, not black, typography with less contrast.
In most browsers, just press “Ctrl” and “+” to make the text bigger; pressing “Ctrl” and “-” makes it smaller. Many apps use Command-plus (+) and Command-minus (−). Or in your browser, choose View, Zoom In.
To change your overall view in Windows, open the Control Panel and adjust appearance/screen resolution settings.