This Adagio is a simple yet beautiful example of Mozart's skill in writing charming and lyrical melodies. It possesses a graceful flow and a serene atmosphere, making it an accessible and pleasing piece.
Focus on producing a clear and singing tone, and maintaining a steady, unhurried tempo.
Adagio in C major is rated Henle level 3 of 9 — late beginner repertoire, roughly ABRSM grades 4–5. In the RCM system it corresponds to about level 3.
You should be comfortable at Henle level 3 — meaning you can already play level 2–3 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of Adagio in C major lasts about 4 minutes.
Adagio in C major is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.