Mozart's variations on the Dutch song 'Willem von Nassau' are an early and charming piece showcasing his youthful genius. The work is characterized by its sprightly character and inventive melodic variations.
This piece requires quick fingerwork and a light, crisp articulation.
7 Variations on 'Wilhelm von Nassen' (K.25) is rated Henle level 3 of 9 — late beginner repertoire, roughly ABRSM grades 4–5. In the RCM system it corresponds to about level 4.
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 7 Variations on 'Wilhelm von Nassen' (K.25) lasts about 6 minutes.
7 Variations on 'Wilhelm von Nassen' (K.25) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.