There are no income limits for traditional IRAS1, but there are income limits for tax-deductible contributions. No, there is no maximum income limit for a traditional IRA. Anyone can contribute to a traditional IRA. While a Roth IRA has a strict income limit and people with incomes above it can't contribute at all, that rule doesn't apply to a traditional IRA.
You can open a traditional IRA through a bank, brokerage agency, investment fund, or insurance company and invest the money from your IRA in stocks, bonds, mutual funds, exchange-traded funds, and other approved investments. You may still want to make a non-deductible contribution, either because you prefer to allow your investments to grow tax-free and defer income taxes or because you want to make a clandestine contribution to the Roth IRA by contributing to your traditional IRA and then converting it into a Roth account. Traditional IRAs don't have this rule, as do other types of IRAs, such as SEP IRAs and SIMPLE IRAs, which are often used by self-employed individuals and small business owners. However, you can still contribute to a Roth IRA and make cumulative contributions to a Roth or traditional IRA, regardless of your age.
Yes, a person under 18 can contribute to a Roth IRA or a traditional IRA as long as they meet earned income requirements and do not exceed income limits. The ability to make non-deductible contributions regardless of income level makes traditional IRAs a valuable retirement savings account that can be converted into a clandestine Roth IRA. Remember that you are also not subject to income limits when you make contributions to a SIMPLE IRA or an SEP IRA; options that are only available if your employer offers them, if you are a small business owner, or if you are self-employed and can open one on your own.