Chronologically, as in by amount of time they've experienced(which is an odd concept, considering that the perception of the passage of time changes from person to person and attitude and such), he might be. In terms of mortal years spend being around humans and shit? Mab is way older. He knew the bastards when they were barely hairless monkeys.
As for how old Mab is, he's a bit older than the solar system, being formed partially by the nebulous disks that formed during the creation of the solar system(since those were some of the first storms and true lightning-places, which is his primary aspect, not magnetism or electromagnetism or anything; those are secondary). He's ~about~ 4.7-5.0 billion years old. It would be hard to say when exactly when he was 'born' or created or whatever since it was a gradual process.
Also, he's by no means the oldest of his kind. There's probably an equivalent embodification of Space or maybe Time who's the oldest among them, followed by things like Entropy, Light, Gravity, Motion, and Dark Matter etc.
Unless the oldest is Void, Creation, and Destruction/Change trio, of course; Preservation & Change, Passive & Active, etc, formed ever so slightly before the Big Bang(time & gods are weird like that; if something's going to be needed soon, they'll form before it really exists.
And, of course, the oldest might be Heat/Flame/Energy, or Substance/Earth/Matter, or this, or that, or the other thing, and so on and so on.
Also, considering Mabarius is 'sleeping', and is way less powerful than he was when he was a god(the power wasn't lost, per say, but it can't exactly be contained even in this large of a sword...)....meh, whatever. Not that big of a difference; Spensir didn't believe Mab was a god, really, in the first place, so this might shake that, but other than that...