I am not sure I understand your question. A 6 weeks the ultrasound is normally very accurate. It is the first time the heart beat can be seen, I am not sure why they can't measure the embryo.
If you had sex 7 weeks and 2 days ago (August 7 is only 6 weeks 2 dys back!), the ultrasound should be measuring a gestational age of around 9 weeks. Ultrasounds does not normally measure fetal age, but gestational age (starting 2 weeks further back than actual conception).
There is no reason to think that it is your last chance. At 39 it can take longer to get pregnant, but your fertility is not gone by a long shot.
Best of luck.