I learned Japanese in 1971 at age 21. Now in 2018 at the age of 68 , I only remember a few words and none of the writing types. I don't believe memory has anything to do with forgetting. It has more to do with daily use. I have no use for Japanese where I live in Maine , so it has faded from my memory. Other skills I learned at age 5 years old that I use each day are still working just fine with no memory fade.
All human beings suffer from memory loss as they age. It might be mild (forgetting of names, forgetting of seldom used words), or it could be serious deterioration of memory as in Alzheimer's disease or Parkinson's disease. But occasional forgetting of words or names is a very human thing, and doesn't necessarily mean a steady, irretrievable loss of language skills.
*not a doctor*