EDIT: you guys have dug up some truly horrible pisstakes :D Thank you for those.
To the serious folk - relax a little. This is Mildly Infuriating, not I'm dying if this doesn't stop. As a non-native speaker I was taught a certain way to use the language. The rules were not written down by me, nor the teachers - it was done by the native folk. Peace!
Yeah it is grammatically correct but most people would say “less than 5 minutes ago” or “less than 50 seconds”, instead of using “fewer than”.
Minutes may be countable but time itself isn’t, I’d say. Generally applies to units: You can certainly count litres but it’s still “less than five litres”, at least when talking about a volume say left in a tank as opposed to things that come in individual 1l containers. The space between that (e.g. 500ml or 1.5l containers) is fuzzy.