Hi
I was advised to make 19 mph and keep it up or get dropped. Oh yes, and at that speed you won't win, break any records or get any street cred!
I should know... I spent a year getting dropped at each race and I was in the 'slow group' age 42yrs!
Don't get me wrong - go for it! It does get better and year on year the strength in your legs improves considerably. Its never too late to go for it! But you must put the miles in week in, week out!

Good luck
Poly