Some people on this thread are using "pensions" (which typically refers to a defined benefit pension based on number of years of service at a company) to also mean 401-Ks (defined contribution). Pensions have become pretty uncommon in the private sector and they're largely unknown in US tech companies at this point.

401-Ks have no underfunding problem. Of course, they expose you to market risk directly.

No, this is your money. 401k (including matching $) is paid into an account you control. You can use it anytime, but if you do before some age (65?) there will be tax penalties.

59 1/2. For Traditional 401(k), you owe the taxes on it and an additional penalty if you withdraw early. You're required to take distributions from it starting at 70 (69 1/2?), because the feds want their tax dollars.

Roth 401(k) plans are less common, you'd owe a penalty but not taxes since it's pre-tax money. No mandatory distributions because they've already gotten your money.

IRAs are the same. HSAs (mentioned elsewhere) are tax-free if distributions are spent on medical costs. For non-medical costs, you owe taxes and a penalty. After age 65 you'd only owe taxes.