I think I have a short vagina. Too late for estrogen or resuming sex?

No. Shortened or narrowed? Dilators and sex work well for enlarging a narrowed vagina, short vagina may require surgery to lengthen but will still allow for some penetration.
No. The vagina can be enlarged with dilation, either through sex or using physical dilators like a sex toy or specifically made vaginal dilators. Vaginal estrogen should be used if needed. This also is very effective at treating bladder symptoms such as urgency, frequency and getting up at night to go to the bathroom.