Ferraro missed 2016 eligibility by two days, so he was simply a second-rounder in his first year of eligibility.
Anyway, age is obviously something that has to be taken into account when scouting players that are 364 days apart but still in the same draft class. But not just to say "this player is old, so he shouldn't be drafted this high."
That "extra year" we get to scout Tkachuk actually helps a lot in figuring out what player he is and what he can become. So much can change between the ages of 17 and 18, so rather than pushing Tkachuk down, I think the extra year is really helping him.
Personally, I've seen more of Tkachuk than Svechnikov or Zadina, so I don't want to make a definitive statement here. But if Tkachuk went 2nd overall, I'd be totally happy with it. He's someone I can see stepping right into the NHL and contributing in a middle-six role.