We have been living in the same house for almost 15yrs. Our landlord did not have us sign a lease when we moved in but now he wants us to. Is this legal?

Perfectly.
Why wouldn't it be.

Well, simply, yes. If you refuse to sign one they can give you notice to vacate. Whatever your state requires - most are 30 to 60 days - for how long that notice needs to be. There's a "sticky" at the top of the Real Estate Law page with a link to each state's laws.

If it's a proper lease it should actually give you more rights and protections than just living there w/out any lease in place. And everything is subject to negotiation. If they're asking you to agree to things that you don't like you're free to not sign it and wait and see if they give you notice to be out.

He can not of course make you sign, you are always free to move. But they can require you to sign and have one to continue living there.

If in writing there are things you don't want, then don't sign and move, or work things out with them