People often say to themselves in life that they should avoid a variety of occupation, and, more particularly, be the less willing to enter upon new work the older they grow. But it is easy to talk, easy to give advice to oneself and others. To grow old is itself to enter upon a new business; all the circumstances change, and a man must either cease acting altogether, or willingly and consciously take over the new rôle.

[Man sagt sich oft im Leben, daß man die Vielgeschäftigkeit, Polypragmosyne, vermeiden, besonders, je älter man wird, sich desto weniger in ein neues Geschäft einlassen solle. Aber man hat gut reden, gut sich und anderen rathen. Älter werden heißt selbst ein neues Geschäft antreten; alle Verhältnisse verändern sich, und man muß entweder zu handeln ganz aufhören oder mit Willen und Bewußtsein das neue Rollenfach übernehmen.]

Johann Wolfgang von Goethe (1749–1832) German poet, statesman, scientist

Sprüche in Prosa: Maximen und Reflexionen [Proverbs in Prose: Maxims and Reflections] (1833) [tr. Saunders (1893), ch. 4, #237]
    (Source)

(Source (German)). Other translations:

People often say to themselves in the course of their lives that they should avoid a variety of occupation, and, more especially as they grow older, feel the less inclination to enter upon any new work. But it is all very well to talk thus and give advice to oneself and to others. To grow old is in itself to enter upon a new business; all the circumstances are changed, and a man must either cease altogether from acting or else willingly and consciously take over his new role.
[tr. Rönnfeldt (1900)]

You often say to yourself in the course of your life that you ought to avoid having too much business, "polypragmosyne," and, more especially, that the older you get, the more you ought to avoid entering on new business. But it’s all very well saying this, and giving yourself and others good advice. The very fact of growing older means taking up a new business; all our circumstances change, and we must either stop doing anything at all or else willingly and consciously take on the new role we have to play on life’s stage.
[tr. Stopp (1995); Art and Activity, Vol. 5, No. 2 "Individual Points," #259]

One often says to oneself […] that one ought to avoid having too many different businesses, to avoid becoming a jack-of-all-trades, and that the older one gets, the more one ought to avoid entering into new business. But […] the very fact of growing older means taking up a new business; all our circumstances change, and we must either stop doing anything at all or else willingly and consciously take on the new role we have to play on life’s stage.
[Var. of Stopp]