Can the sorcerer's Metamagic option Twinned Spell be used on the Eldritch Blast cantrip?

Question

I was just reading this on an online max-DPR build. There were some who thought using the Metamagic option Twinned Spell on the warlock's Eldritch Blast cantrip was illegal, and some who thought the opposite.

The Twinned Spell option for the sorcerer's Metamagic feature reads (PHB, page 102; emphasis mine)

When you cast a spell that targets only one creature and doesn't have a range of self, you can spend a number of sorcery points equal to the spell's level to target a second creature in range with the same spell (1 sorcery point if the spell is a cantrip).

Can Twinned Spell be used with Eldritch Blast?

Answer 1

Only when the caster is below 5th level

The errata for the PHB has clarified the restriction further from the original printing:

To be eligible for Twinned Spell, a spell must be incapable of targeting more than one creature at the spell’s current level.

By default, eldritch blast does not have a range of self and is capable of targeting only one creature. It is thus eligible to be twinned.

However, the spell becomes capable of targeting more creatures once the caster reaches level 5:

The spell creates more than one beam when you reach higher levels: two beams at 5th level, three beams at 11th level, and four beams at 17th level. You can direct the beams at the same target or at different ones. Make a separate attack roll for each beam.

Thus, once the caster reaches 5th level and above, they can no longer twin eldritch blast.

Note that eldritch blast (and all other similar cantrips) scale with character level not class level as confirmed (unofficially) by Jeremy Crawford on Twitter. (See Do Cantrips use your character level or class level? for more discussion about that)