The derivation from *kap- “to fall” is semantically very difficult. By the way, the Bactrian word is in fact καβογγο /kabung/ < *kapuna-ka.It descends from the word seen in Zoroastrian Middle Persian as <khwbn> /kahwan/ and in Manichaean Middle Persian as <qhwn>. Cognates include Parthian <kfwn>, Balochi kwahn, etc., Bactrian καββογο, and Khotanese kuhana-.
According to Bailey, Dictionary of Khotanese Saka, p. 64, the root etymology is Iranian *kap-/*kaf- "to fall, befall, strike down" (Manichaean MP kp-, kf-, qf-, Kurmanji ketin, kev-, Zazaki kewtiş, kewn-, Balochi kapt-, kap-, kab-, all "to fall", but without verbal reflexes in New Persian). Iranian *kafvana-, as Bailey reconstructs it, would be *"fallen" > "old".
The suffix –wana can form deverbal abstract nouns, as in Avestan āfriuuana- “blessing”; several Sogdian examples are cited in Gershevitch’s Grammar par. 1084. But it does not seem to form adjectives.I am curious about the suffix *-vana- in Bailey's proposed *kafvana-. Is there a good example of a parallel deverbal formation with *-vana- to be seen in any other Iranian word? I'm curious because Vedic -vaná-, forming agent nouns and adjectives, is not common suffix.