Ok, here comes the pilot answer. Take-off is basically from the time you start the takeoff roll until you are at a certain altitude above the ground. That altitude is not defined. At the place I am flying for right now, it is 500 feet.

Landing is within 300 feet of the ground where I'm flying. Once again the altitude is not specifically defined.

All commercial air carriers must tell passengers when to put their seats upright (for approach and landing). Every airline I've ever flown on does this at the "double-chime" which indicates 10,000 feet above the ground. That is also the altitude at which portable electronic devices must be turned off.

Now switch that to takeoff. There is also a "double-chime" passing 10,000 which indicates PED's may be used. I would say this is also the altitude at which seatbacks can be reclined. This is what I normally do when I'm riding as a passenger.

The seatbelt sign is not used to indicate anything but the seatbelt. There is a specific safety risk that requires you to remain in your seat, but that does not apply to your seatback.

All of that said however, I would still not say anything to someone who did recline after takeoff but below 10,000 feet. For one thing, it's extremely rude. For another thing, PED's are really the most important issue during climbout.

Hope that clears it up.