He's 6'6" and has had alcohol issues in the past coupled with injuries. He's not in top physical shape obviously, but he's still performing at a consistent level good enough to rely on for a lot of wins/team wins. And it's baseball, very different than football. Not many 38 year old footballers now.
Either way, back to Lukaku. He can go wherever he wants to during the off-season. However he's accountable like every other current United player to get their training in and be ready for the upcoming season because they are United players until further notice. Ole said that each player has their specific training sheets and goals to hit in lieu of the new season, i.e. they need to be prepared to hit the ground running and if they aren't, it's their fault, nobody else to blame.