Sorcery Points may be used for Metamagic you know, no matter your level.
Consider my favorite Metamagic, Quickened Spell:
When you cast a spell that has a casting time of 1 action, you can spend 2 sorcery points to change the casting time to 1 bonus action for this casting.
Observe that Quickened Spell does not care where the sorcery points came from. If you know the Metamagic, then you can use your sorcery points on it, because the Metamagic does not discriminate.
In contrast, other sorcerer features do discriminate, because of what is written in the feat:
You gain 2 sorcery points to spend on Metamagic (these points are added to any sorcery points you have from another source but can be used only on Metamagic).
The points from the feat can only be used for Metamagic. The points from the class have no such restriction. Sure, sorcerers usually do not have access to Metamagic until 3rd level, but 5th Edition has a specific-beats-general rule:
Remember this: If a specific rule contradicts a general rule, the specific rule wins.
Generally, sorcerers don’t have any Metamagic until 3rd level, but the class description doesn’t say they can’t use Metamagic before then, the only barrier is that they don’t know any. Metamagic Adept overcomes this barrier by teaching you the Metamagic, providing a specific rule allowing you to use Metamagic, overriding the general rule that 2nd level sorcerers don’t know any Metamagic.
Another helpful way to think about this is:
Why would being a sorcerer put limitations on the Metamagic Adept feat when other classes would have no such limitations?
A Variant Human warlock could take Metamagic Adept at 1st level and immediately have access to the sorcery points and metamagics gained through the feat, so we should not expect that being a sorcerer would prevent that availability, even at 1st level.
