Agree with STS.
With very little game balance, if the story is good and fun being had, then do what makes sense for the players.
Leveling is a grind at the higher levels, often testing the player's patience. So if a player wants to play the grizzled old timer, start at the level they want but have them give a good background story to explain why. Old timer muck farmers might never see 5th level, so a 12th level mage better have good stories to tell while chugging back cans of Dr Pepper. This allows the player to craft the character they want, at the sacrifice of the true emotional investment in their character that comes with gameplay. I'm not against playing higher level characters, but I do believe it can rob players of the surprise that can come with organically grown characters, but folks will do what folks will do.
Powers, which as pointed out are often frontloaded, determine what degree of challenge the players need to overcome. A 1st level LLW should be facing more significant challenges than a 1st level muck farmer, simply because if they don't their powers are wasted and the players might get bored. They chose to be a LLW for a reason...
I've found these two concepts come into play when someone suggests using a multiclass character. I'm not opposed to multiclassing (within reason and bounds of a story) because I've found it to offer two significant gameplay advantages:
1) If the work is done on building out the backstory, multiclassing gives players access to a diversity of powers and skills that helps shape the character in many ways like they themselves are evolving as they get older. Younger players might not get this and see multiclassing as simply munchkinism, but more... experienced players know that a lot can happen in 20-30 years and people IRL go through phases and change careers or even fundamental philosophies and ways of living.
Responsible multiclassing allows their characters to do the same thing as they age too, adding that little bit of realism.
2) The rules are a bit unforgiving when it comes to leveling skills as they keep levels generally low for a much longer time, particularly if multiclassing more than once, representing a "jack of all trades, master of none" gameplay experience. Often players argue that skill rolls are a distraction when their skill is >90% if playing a high level character, so if most of their skills are frozen at 30-50% from multiclassing, they can be more engaged due to the fear of the failed roll. This tension means the players have to be smart about finding buffs, for example seeking out that illegal pre-rifts text book to get a small % bump which could be an adventure in and of itself.
And just so we're clear, I'd almost never condone a backstory of a 9th level dbee mind melter becoming an initially unwilling 11th level partial cyborg that retired to become an 8th level BM merchant at the ripe old age of 387. That degree of munchkinism is on a whole other level...
