I had the same issue when I would do the same thing but in my family tree software (Family Tree Maker). For that particular software, I think the parents' ages had to be a minimum of 13 years older than the first child. However, in my experience with the families I have dealt with (Canadian/Irish mostly), a trend emerged quickly with regard to how old people were at particular milestones. For example, most girls married between the ages of 16 and 21, and I don't think I have found a man yet who married before the age of 18. Typically, the first child was born about 2 years after the marriage, with subsequent children occurring every 2 years after that.
Generally, my estimates look like this:
If the known or estimated DOB for child = 1850
Then the estimated year of marriage for parents (less 2 years) = 1848
And estimated year of birth for mother (less 16 years) = 1832
And estimated year of birth for father (less 18 years) = 1830
I would say if your grandfather was born in 1817, then I would estimate his parent's marriage in 1815, his mother's birth about 1799, and his father's birth about 1797.
I hope this helps!!