They wanted to create their own show.
Series co-creator Greg Weisman talked about how they wanted to do something different with Young Justice which wasn't Teen Titans 90s Young Justice, or JLU.
We were going to reset the DC Universe and start young with everyone under 16...
We went through something like 50 dc teen heroes to build a team... Tried to get a diverse mix and put together a group that worked for us.
Keeping in mind that our Earth the heroic age is new... If Superman has been around for ten years and Batman has been around for nine years, if you are going to do Robin, then what Robin are you going to do? You are going to do Dick Grayson, because he was the first Robin... That is when the universe began.
He goes on to say that they designed a new Aqualad, who isn't Garth, and the concept was from the ground up.
“Phil Bourassa, who is our lead character designer on the show, did the original design for Aqualad and Brandon and I basically created him; Geoff [Johns]came in and really liked the character and we talked about him a lot,” Vietti recalled. - cbr.com
In another interview he had this to say about the characters.
In some ways, this series is an adaptation of the entire DC Universe. But the key word is "adaptation". I strive (as does my "new" producing partner Brandon Vietti) with any adaptation (again cf SpecSpidey) to stay true to the spirit and intent of each and every character. But that doesn't mean there won't be changes. - Station Eight - Ask Greg
For the age question, from the same interview.
Brandon and I chose WALLY WEST as Kid Flash over Bart Allen as Impulse in part because it is EARLY DAYS in this particular DC UNIVERSE. And Wally was the original Kid Flash, just as Dick Grayson was the original Robin.
Here is the interview he gave at San Diego Comic Con (source for first block of quotes)
As to Robin's age.
Station Eight's Ask Greg
Question: at what age did robin lose his parents?start being robin?
Greg: ASKED AND ANSWERED. (Age 9.)
So he started as Robin at age 9, in Young Justice Season one he is age 13 (having been operating as Robin for 4 years with Batman). In season two he is age 18 as Nightwing.
Earth-16 is a young place.