How would you need to change the game to keep it fun for everyone if you wanted all pcs to start at 1st level, even if the party was significantly higher level?
Back in my AD&D days, this was what we always did. Start a new character and join in, that's 1st level.
The whole thing about starting PCs at the level of the party, or one level behind or whatever came around in 3e, for the groups I played with anyway.
It was viable compared to later editions of D&D for several reasons:
1. The XP handed out in higher level games meant that lower level PCs would level up very rapidly. In the first D&D campaign I ever played in, I joined a game with 9 PC's averaging about 12th level. I leveled up once per session for the rest of the campaign, when my PC was 14th level.
2. Damage totals were lower, especially melee damage, meaning that PC's were less likely to be killed in one hit even in higher level games. My first PC, the one from the above example, was a Cleric who started with 8 HP, and since the Death's Door (aka -10) rule was in effect, she wouldn't die until she hit -10. In practice this meant that most times monsters hit her she would be at critically low HP or knocked out, but she wasn't killed because the number of monsters that could do 18 or more points of damage in 1 hit in AD&D was pretty small. You had a non-negligible chance of not being killed outright by a min-strength 5d6 Fireball or Lightning Bolt, and you could probably soak up a casting of Magic Missile without dying. This meant my PC was getting knocked out a lot, but I was still getting lots of XP and certainly enough to level.
3. We didn't use miniatures, thus in a more narrative-oriented combat it was accepted, even expected for the less powerful PC's to state that they were standing back out of the fight and just running up briefly to heal or something, or staying in the middle of the group if it was a large melee. The DM accepted this and the monsters generally attacked the fighter-types that were attacking back unless it was a huge mob of monsters (like an orc horde or waves of zombies or something) where there were enough to attack everybody.