First, let me give a slow clap for all the uses of the word "punitive" in this thread. It sounds as though those of you using it are using a playstyle that considers not giving a bonus out as equal to a penalty? That's fine, nothing wrong with that, but your approach is not objectively superior, and your playstyle isn't for everyone. Starting at first level is neither 'punitive' nor (and this was the best one in the thread) a 'house rule'- in fact, for most of D&D's history, that's how the game was played. Moving on from that is fine, if you like it; but that's a playstyle choice, not- I re-emphasize- objectively better play.
Second, let me point out that, given bounded accuracy, being low level doesn't make a pc a non-contributing member of the group. In contrast, bounded accuracy means that the first level guy is actually able to help higher level groups out.
Obviously some of you (@KarinsDad, in particular) don't like this and wouldn't want to do it and might even walk away from a good game with this rule in play. That's fine. There are others, equally obviously, who do like it, who would love to play in this style and see the way it shakes out as a solid positive. That's also fine.
I'm not trying to persuade anyone here. Whatever hard-to-read tables people post, it isn't going to persuade me to change my tune on this. What would change my tune is running 5e this way for a while and discovering that it doesn't work and it's not fun for everyone. The doubters in this thread seem to feel like running an ES@1st game would automatically be no fun for the new guys. Given that I did exactly that from about 1981 until, what, 2000? or whenever it was that 3e released, I assure you that it CAN work.
Now, if I start throwing beholders and giants at the party left and right just because the highest-level pc is 10th level, yeah, that could ruin the fun for the low-level guys. But I don't really do that. A beholder is going to be in the same place whatever level the pcs are- probably in its lair, deep in a nasty dungeon/Underdark/whatever. And 5e makes a mission against goblins a valid adventure even if part of the party- or hell, even if the WHOLE GROUP- is 10th level. So why all the assumptions that every challenge the pcs face will be suitable only for the highest level pcs? Why assume that the damage output of every challenge they face will be suitable for the 10th level guys and capable of one-shotting the low-level ones?
Not only that, why assume that the low-level guys are going to run up into melee against a giant if they do encounter it? Ranged attacks, opening the locked door that the pcs need to get through, fighting the orcish minions- there are tons of ways for an encounter to be fun and challenging for low and high level pcs together. Seriously, bounded accuracy opens this up.
One more thing before I rebut Elric's four-point post above, and maybe I should have mentioned this upthread, but another thing really enabled by 5e is troupe style play, where everyone has more than one character and picks the right one for a given mission. If the low-level guy is having too rough of a time of it, the other players (not characters, players) might well agree to use some of their other pcs for a while, allowing the low-level pcs to catch up a little before mixing with the high-levels. All it takes is player-to-player communication.
Starting PCs at level 1 in a medium-high level group means that: 1) new players won't want to join, 2) old players who are tired of their characters will either begrudgingly play on or simply drop out, 3) no one will want to risk anything, 4) when characters die, or even are threatened with death, the issue will be extra contentious (if only you had healed me/attacked more intelligently, I wouldn't be reduced to being a caddie for the party now!).
Decades of experience belies this... dare I say it... theorycrafting.
1. I literally ALWAYS have a waiting list of people who want into my games. I have had this waiting list since the late 80's at least. Since then, the only times I have not had people clamoring to get in has been when I recently relocated. Heck, at the moment there are two new players waiting for us to start 5e to get into the "Alpha" game, and I'm starting an entire second group up with all different players because I don't want to try to cram all the playerss into one room around one table. So, nope- new players definitely want to join my group, and always have, including when I was running ES@1st before.
2. Old players who are tired of their characters make secondary pcs and then switch back and forth over time. Heck, I've seen a player semiretire his fairly high-level (for 2e, anyhow) druid and start up a bard... who he played from 1st to about 7th before switching back, with only one or two druid-related adventures in the middle. So, nope- old players don't play "begrudgingly" or drop out unless life forces them to.
3. PCs take plenty of risks. They also take pains to avoid unnecessary danger, if they are smart, regardless of level or party composition. Frankly, this assertion is pretty ridiculous. PCs didn't refuse to take risks in Basic, 1e or 2e; why would they now? So, nope- pcs still take plenty of risks, albeit cautiously, if that makes sense. So, nope- the pcs don't become overly cautious and risk-averse.
4. The most contentious pc death I have seen was in my 4e game, just a few sessions ago, when the epic wizard died through an unexpected power of a monster while on a mission for the epic vampire. The wizard player keeps telling the vampire player that she killed his character, but it's all in good fun. Anyway, the point is that ES@1st has never caused real contention that I've seen, but "Damn, now I have to spend six hours building a new character at 25th level" has. So one final nope- the contentiousness of death really isn't an issue with the people I game with. (If someone gets butt hurt that easily by something in a game, especially by something pretty common and often self-inflicted, like pc death, I probably don't want them in my group.)