When you introduce the system, of course, you have to give people credit for all their past income tax paid. Most people of working age will have overpaid and will get a windfall in the form of a huge one-off tax credit, which will compensate them for the higher tax they will pay later in their lives.
If letting people hold onto their money early in life and give it to the government later is considered to be undesirable public policy, it would be possible to even out the tax by including tax deductions based on age. For example, when computing your lifetime taxable income, subtract a "lifetime personal exemption" of $4k times your age. This causes no problems other than that your annual tax bill could potentially be negative (if you earned less than $4k that year).
When you introduce the system, of course, you have to give people credit for all their past income tax paid. Most people of working age will have overpaid and will get a windfall in the form of a huge one-off tax credit, which will compensate them for the higher tax they will pay later in their lives.
If letting people hold onto their money early in life and give it to the government later is considered to be undesirable public policy, it would be possible to even out the tax by including tax deductions based on age. For example, when computing your lifetime taxable income, subtract a "lifetime personal exemption" of $4k times your age. This causes no problems other than that your annual tax bill could potentially be negative (if you earned less than $4k that year).