You can start to take qualified distributions from your solo 401k when you reach the age of 59½.
Early withdrawals before the age of 59½ are subject to a 10% early distribution penalty, plus income taxes on the amount withdrawn.
Qualified withdrawals after the age of 59½ have no 10% penalty, but income taxes are applied if withdrawing from pre-tax accounts.
Withdrawals from your pre-tax solo 401k will be taxed as regular income.
Withdrawals from your post-tax Roth solo 401k will be completely tax-free.