What is the age interval for a young growing horse in PC-Horse? In the help screen the "age-bar" only extends to 24 months?
In PC-Horse, a horse is defined as a young, growing horse from birth until
36 months of age (3 years). Calculations in PC-Horse use this as the age interval for equations affecting young growing horses. If your horse is between 24 and 36 months at the time when you create it in PC-Horse, you must provide the date of birth in the birth date-field.

I tried to import horses from my old programs. But it went wrong. Only certain horses were imported?
It may be because you have named your old horses in such a way that your current Windows version cannot handle them as valid file names. For example, a filename containing any punctuation (.,,:) will cause a problem.