Generally speaking… when you turn 65. However, every plan is different and some allow you to begin distributions much earlier. Possibly as early as 55. Pensions payouts are generally taxable as current income.
Some pensions allow a lump-sum distribution to you directly or as a roll-over. A roll-over can be made into an IRA or an annuity. Be careful as a lump-sum distribution may be taxable, might bump you into a higher tax bracket, and can incur the 10% excise tax.
Lump-sum or a monthly payout, taking an early distribution may affect how much you receive. Speak with your benefits administrator or a fee-only advisor as the options can be complex and there are usually no do-overs. Make a mistake and you are stuck with the consequences.