No, I do not know at what age this becomes a real issue.
BUT as a mother of seven I can tell you that each child developes skills within their own time frame.
It bothers me a little that you are so concerned. I lay that to reading too many books about children.
In my day we had NO books ( until Dr. Spock came out a ruined an entire generation of kids)
We also had extended families, Mums, Grams, Aunts, cousins, neighbors with lots of kids, they were a five minute walk and a cup of tea away.
You new Mum's regretfully do not have that option.
BUT you do have Mamasource with many fine respondents.
I have had children run as early as four and a half months...YES IT IS TRUE..altho I always attributed it to his severe hyperactivity rather than a develpmental ability..my latest walker was nearly 16 months. He did not want to fall and like your daughter was extremely cautious. He too, was a bit slow , a lot slow, with gross motor abilities. He now, at 35, is a well known boat builder ( go to Lowellbrothers.com) I am talking about Jamie.
When he was five I was told he was so severly learning disabled and dislexic he would never be able to mainstream in regular school. I thought he was brilliant and I needed only to find out HOW to teach him. He went to school, I found a way to teach him to read, his junior year at high school he recieved a letter from the superintendent and principal commending him on attaining honor level grades for his three years of high school.
SOOO...check with your doctor next visit, if he says there is nothing wrong, accept that she is a bit slow in that part of her development.
You can , with docs advice, find some sort of exercise and activity that will help her along. Maybe a playground, make it a weekly outing for all the kids? A little trampoline? A large ball? Dancing? At three she is old enough to start tap dancing lessons.
Dont worry hon..each child has its own agenda and just because some eighteen month old is doing somersaults doesnt mean your daughter is handicapped...just a bit different.
Tincture of time cures many ills.
God bless
Grandmother Lowell