A general rule is that you can live indefinitely if you withdraw 3-4% of your investments annually to pay living expenses. There are calculators out there (Google 'FIRECalc') that work out the odds of success (i.e, not going bankrupt) with various starting points and spending rates. At 3% there's about a 95% chance of success over a few decades, based on the past performance of the financial markets.
Let's say you spend $5k/month normally, as a round number. That works out to between $1.5M ($5000x12/3%) and $2M ($5000x12/4%) to sustain your living expenses indefinitely with no further income.
This dovetails nicely with the conventional wisdom that over 30s is unemployable in tech fields.
So either retire penniless in your 30s and never work in tech again, or retire with $5M in the bank in your 30s and never work in tech again.
Also if your gross is say $120K and you keep about $90K of it and save most of it to retire, living off maybe $30K which is more than most Americans get, 5e6/30e3 = a mere 166 years of retirement, assuming your investments return about the real world inflation rate, which is a fairly realistic goal. Of course $5M in the bank and $60K/yr invested means you will have to start your savings program at age -60 or so if you want to retire in your 30s.
Its probably a lot simpler to just contract like my Dad did. Do your thing when you need the cash, and when you don't need the cash, don't work for awhile. Nobody wants the programming contractor to advise the board at the end of a 16 hour day, especially when they see your hourly overtime rate.
I work at a startup with a development team of 9 people. Of those, one is in his 30s, two are in their 40s, and two are in their 50s. The rest, myself included, are late 20s.
Just because some parts of the tech industry fetishize young developers doesn't mean all of them do.
That depends entirely on what sort of lifestyle you're trying to sustain.
There shouldn't be much difference between the level of savings needed to retire at (say) 35 and (say) 45. In either case, you want enough that with low-end-of-plausible stock market growth, you're taking out less than growth is putting back.
There's some research around what level of growth it's safe to assume; how much you trust it depends on whether you think the past behaviour of the US stock market is a good predictor of the future behaviour of whatever you invest in. I suggest (but this is Not Financial Advice and I am Not a Financial Advisor) working on the basis that you can probably get ~2.5% of your investment out per year pretty safely. (Usually it'll grow more than that, even after inflation. That cushions you a bit against cases where bad things happen. I assume you are willing to trim your lifestyle back a bit in bad times.)
That means you need about 40 years' worth of spending in your savings. That would be $5M if you are spending $125k/year. In my opinion (about which you need not care), if you are spending $125k/year then either you are earning so much that saying $5M isn't going to be a big challenge for you, or else something is amiss with your priorities.
Let's say you spend $5k/month normally, as a round number. That works out to between $1.5M ($5000x12/3%) and $2M ($5000x12/4%) to sustain your living expenses indefinitely with no further income.