Young Justice is notable for numerous reasons, but one of them was that it’s one of the few productions to have the then five different versions of the Flash (Jay Garrick, Barry Allen, Wally West and Bart Allen) in it at the same time.
However, in the DCAU, it was implied that the three older versions of the character were implied to exist… sort of. In the episode of Justice League Unlimited, Flash and Substance, we get a look at the Flash Museum, and amongst the exhibits we see both Jay Garrick’s helmet and Wally’s Kid Flash costume… Yet there’s no indication that Barry Allen existed.
It could be that Wally somehow managed to independantly get superpowers some time before the events of the Flash’s debut DCAU episode (the Superman: the Animated Series episode Speed Demons), and was Kid Flash before just becoming the Flash when he was older. After all, in Young Justice Wally got his powers by deliberately recreating the accident that got Barry his powers, maybe something similar happened in the DCAU?…
…Or the Flash that appeared in Speed Demons, who wasn’t voiced by JLU's Michael Rosenbaum but Charlie Schlatter instead, WASN'T Wally but Barry. And that Wally's mentor died at some point prior to Justice League, explaining why Wally seems a touch more immature than you’d expect for someone who’d been a public superhero for some…ten or so years roughly? If we take the “everyone ages roughly in real time” thing that the DCAU seems to have.
And this is why I still think that the Teen Titans cartoon is canon in the DCAU. Aside from the Teen Titans VAs for Speedy and Kid Flash play Speedy and the Flash in JLU, we have this.