In order to keep the game challenging and fun at every level, you should set a DC that's appropriate for the level of the characters, using the Difficulty Class by Level table. Decide whether you think the task should be Easy, Hard, or somewhere in between for the characters, and use the DC from the appropriate column of the table.
However, don't throw all sense of realism out the window. If a simple wooden door in a dungeon, stuck from years ofdisuse, requires a DC 10 Strength check for a 4th-level character to break down, there's no logical reason the same door should require a DC 21 Strength check for a 24th-level character to break through. When characters have reached 24th level. they don't need to worry about simple wooden doors stuck shut in dungeons. They can either blow through such doors with ease, or you can present them with barred adamantine doors that do require DC 21 Strength checks.
