Would not be involuntary — a person can choose not to participate, and that means no voting, no govvie job, no public assistance, and no running for office later.
Change your mind when you’re 40? Cool. Go plant trees for a couple of years in one of your State forests.
No thanks. It was a bad idea when Robert Heinlein proposed it, and it’s still a bad idea today. We simply have no legitimate need for the vast horde of make-work “government servants” that you and Robert want to create. Your proposal also inverts the relationship between We the People and our government. It serves us. We do not serve it.
In the case of voting, this type of restriction is as equally unconstitutional as involuntary servitude is unconstitutional. Likewise there is no constitutional requirement to have been a former federal employee to run for President, Vice President, or Representative. You would again have to amend the Constitution.
If your state doesn’t demand your service then the feds have no business demanding it either. If your state does demand it then the feds have no need of it.