When can my son use the umbrella stroller?

He's 3 months and has no problem holding his head up. He definitely can't sit up on his own (too wobbly but he loves to sit in his seat) and I can't say that I would be comfortable with him falling asleep in an upright position unsupervised.

I know that they say six months is the minimum for a travel stroller but is that the actual minimum (vs a minimum they put for liability reasons) or is there some other yardstick to gauge whether or not a baby is ready?