The best age to start handling a weanling is not actually at weaning — it is at birth. Foals that receive competent, calm human handling from their first hours of life are dramatically easier to wean and work with at every subsequent stage of training, because the neurological window for forming secure associations with humans is widest in the first days and weeks of life. Foal imprinting work pioneered by Dr. Robert Miller demonstrated that foals handled systematically in the first hours after birth can be desensitized to an enormous range of stimuli before they have the strength or speed to resist effectively.
If early imprinting was not done, the next best window is as early in the foal's life as possible while it is still dependent on its dam. A foal that has been caught and handled regularly during the first months of life — scratching, haltering, picking up feet, accepting spray and clipper sounds — arrives at weaning with a foundation that makes the social and training stress of separation far more manageable.
For foals that have received little human contact before weaning, the weanling stage itself — typically four to six months of age — is still a very good time to begin. Weanlings are small enough to manage safely with appropriate handling skill, curious enough that positive early experiences create lasting positive associations, and young enough that whatever handling habits they form now will shape their attitude toward humans and training for the rest of their lives. Waiting until a young horse is a yearling or two-year-old to begin basic handling is a significant missed opportunity that always shows up as more resistance and more remedial work down the line.