Addressing your options in order:

1. A younger version of himself.

It depends on how you interpret the rules.

Note the description for True Polymorph (emphasis mine):

"Choose one creature or nonmagical object that you can see within range. You transform the creature into a different creature, the creature into an object, or the object into a creature (the object must be neither worn nor carried by another creature)." [PHB 283]

As Dokebibul noted in the comments, the meaning of this is open to interpretation. In my opinion, a younger version of oneself does not count as a 'different creature', so this cannot be done.

For 2 & 3:

2. A younger version of any other creature 3. A long-lived creature (dragon, elf, etc)

Yes you can. The spell never makes any specific claims about the age of a creature, only its challenge rating.

If you turn a creature into another kind of creature, the new form can be any kind you choose whose challenge rating is equal to or less than the target’s (or its level, if the target doesn’t have a challenge rating). [PHB 283]

So you could turn yourself into an elf or similarly long-lived creature which is also a level 20 wizard.

If you wanted to turn yourself creature like a dragon with the intent of doing this repeatedly, you need to be sure to note the spell casting abilities of your new form. From the spell description:

"The creature is limited in the actions it can perform by the nature of its new form, and it can’t speak, cast spells, or take any other action that requires hands or speech unless its new form is capable of such actions." [PHB 283]

If you choose a creature that can innately spellcast (or it has hands and can speak), then this isn't a problem.

Yes you can. The spell never makes any specific claims about the age of a creature, only its challenge rating.

If you turn a creature into another kind of creature, the new form can be any kind you choose whose challenge rating is equal to or less than the target’s (or its level, if the target doesn’t have a challenge rating). [PHB 283]

So you could turn yourself into an elf or similarly long-lived creature which is also a level 20 wizard.

If you wanted to turn yourself creature like a dragon with the intent of doing this repeatedly, you need to be sure to note the spell casting abilities of your new form. From the spell description:

"The creature is limited in the actions it can perform by the nature of its new form, and it can’t speak, cast spells, or take any other action that requires hands or speech unless its new form is capable of such actions." [PHB 283]

If you choose a creature that can innately spellcast (or it has hands and can speak), then this isn't a problem.