You can start hiking with a baby almost right away. For running and biking, they need turbulence-ready neck muscles (that is, the ability to control their heads, with a helmet for biking), which generally develop around eight months to a year.
HIKING For short outings with babies up to three years old, we prefer Ergobaby's Performance carrier ($135; ergobaby.com). It's ultralight and breathable, and you can wear your little monkey on the front or in back. For longer outings, we like Kelty's Pathfinder 3.0 ($280; kelty.com), with a sunshade, kickstand, enough cargo capacity for snacks and layers, and an adjustable torso length for easy switches between mom and dad.
RUNNING BOB's Revolution SE ($449; bobgear.com) is the lightest and most versatile jogging stroller available. The suspension takes the jolt out of bumpy roads, it can fit infants as well as kids up to 70 pounds, it collapses down for easy transport, and with the optional weather shield ($55), it fends off wind and rain.
BIKING For shorter rolls with tots weighing less than 35 pounds, iBert's GF2 Safe-T-Seat ($110; ibertinc.com) sits right behind your handlebars, so your child gets a view of the road instead of your backside. For bigger kids and longer rolls, we like trailers. They're safer than bike seats because they're lower to the ground, they offer more protection (most have what is essentially a roll cage), and they're easier to maneuver. Burley's D'Lite 2013 trailer ($629; burley.com) has all the bells and whistles—sun shield, seats for two, sturdy shocks—and plenty of room for groceries.
MULTISPORT The Transformer-like Chariot Sport Series Carriers (from $425; chariotcarriers.com) elegantly convert in minutes from bike trailers to jogging strollers to—get this—cross-country ski trailers (conversion kits from $75).