You can use the Obama birth certificate turn around as counter point and that was definitely less than 10 years.
President Obama is on video, audio, Twitter, and just about every other form of media saying things that subsequently were proven false. A brazen example of that is the infamous line, "If you like your plan you can keep your plan." which he knew to be false (and Ezekiel Emanuel, architect of much of the ACA had apparently laughed at as impossible).
I don't mean to single out President Obama as it's really all Presidents, or more generally politicians, that do it. It's not unique to Trump, didn't start with him, nor will it end with him.
Just the refusal to admit them would be bad enough, but since he's gotten away with it just about every time, it shows a worrying trend of there being no accountability. Combined with the frequency, the problem is just multiplied.
I find a politician that can't seem to be held accountable for his actions and that trades on this for more political power very troubling.
Telling people you are going to strengthen whistle blowing laws to empower workers, then prosecuting a record number of people under the Espionage act is closer to a damaging lie.