Like I know most PC's from levels 1-4 will have a +4 or a +5 on average to their skills, attack rolls, et cetera. (And saving throw DC's will naturally hover around 12 or 13--8+4 or 8+5 respectively.)
Ergo in the early levels, they can pass DC 5's reliably, DC 10's often, DC 15's sometimes, and DC 20's and up are effectively incredibly hard save for characters who go all-in on that sort of thing (expertise, guidance, bless, enhance ability, et cetera).
Conversely, I know most PC's will walk into level 10 content with a +9 on average, so a DC 10 suddenly becomes effortless, a DC 15 easy, a DC 20's a coin toss, and DC's 25 & 30 difficult and near impossible each respectively. (Again: Barring expertise, guidance, enhance ability, et cetera.)