Ruby method that returns an age from a date of birth
Make an additional function  to handle plural

In-lining the extra logic to handle the plural is messy, I suggest a small separate method:

def plural(word, value)
  "#{value.to_s} #{word}#{value &gt; 1 ? 's' : ''}"
end


Reduce nesting by returning early

If you write 

if dob == nil
  return "No Date of Birth"
end


You can reduce nesting in the rest of the function and highlight the handling of the special case.