I have to disagree with that; I'm fairly young(mid twenties) and started watching Scooby Doo when I was 11 or 12 years old, and I knew the shows were old, also I knew that the show debuted in 1969 when I became a fan of it so it didn't really bother me too much. Plus my mom works with elementary school kids and some of them love Scooby Doo; I think it really doesn't matter how old a cartoon is, if it's good kids will watch it.
Also, as for WB not wanting kids to know how old the show is; this is 2010, kids can look up information about cartoons online any time they want; I think most kids(like I was when I started watching Scooby a lot) look up info about Scooby and are suprised that it's been on for so long, I think most kids reactions when they find that out are: "1969? Boy I never knew this show was on so long, it's actually pretty funny" or "I never knew old cartoons were so good".
Back before He-Man and the toy based cartoons took over the airwaves along with Disney shows, some of the more popular cartoons on TV were older cartoons; like Looney Tunes or Popeye, those cartoons at the time they were broadcast on various TV stations around the country were probably 20, 30, heck even 40 years old; the reason they were so popular with kids is because they were fun, not because they were old.
Shows like the Flintstones and the Jetsons are almost 50 years old and people(including some kids) still watch those shows because they are funny; basically it doesn't really matter if a show has been on for 40 years or not, if it's fun and a good show, it will get ratings and people will watch, it's been that way for as long as I've watched TV and it will be that way for years to come.