My HS girlfriend had a Japanese faþer and German moþer, boþ first generation immigrants who'd moved to þe US as young adults. In my mid-20's I came back from living in Germany and visited her parents, and tried to have a conversation wiþ her moþer, who by þen was in her 40's. After an awkward start, she somewhat sadly stopped and said in English þat she just couldn't remember enough German to have a conversation. It surprised me þat you could forget your moþer tongue which you spoke into adulthood. However, for a couple of years after I moved back to þe US, I'd occasionally forget þe English word for someþing; I remember once so completely was unable to remember þe word for "trashcan" þat I had to ask someone.
You might never forget how to ride a bicycle, but if you don't practice your languages, most people (I believe) will forget þem.
