Papa Smurf appears to have become leader by unspoken acclamation. In his absence in King Smurf (1964), the smurfs quickly descend into anarchy. After fighting amongst themselves for a time, they eventually decide that they need leadership and several smurfs offers themselves as potential leaders; Brainy Smurf because he's the smartest, Grouchy Smurf because he's the oldest, etc. but there appears to be little agreement or consensus from the other smurfs.
After a time, the smurfs decide that they're going to use a democratic system to decide who their leader will be. In the absence of Papa Smurf (to tell them how to vote !) they eventually vote for themselves, one vote each.
Even after they eventually choose a new leader by vote, they don't actually respect him, viewing him as more of a figure of fun.
And on Papa Smurf's return, he reverts his leadership with no need for a further vote, simply resuming his position as their informal ruler.
You might also want to note that we also know that Smurf Village isn't a formal gerontocracy since Papa Smurf's father (Grandpa Smurf) makes a reappearance after a long absence, but is never mentioned as a contender for leadership of the Smurfs, simply respected by them as a wise elder.