Yes, it is true that “either twin could be considered to be moving at close to the speed of light”. And it is also true that so long as neither of them accelerates each will think that the other is ageing more slowly. But this is no paradox as there is no way for them to get together to see who is right. That can only happen if one or other of them turns around. And in that case whoever turns back towards the other will end up thinking that the other aged so much more quickly during the turn-around that when they get back together there will be a well-defined answer to the question of who is indeed younger and by how much. (They will not agree on the exact pattern of “when” the different ageing rates occurred, but since there is no absolute definition of “when” for events that do not happen at the same place, that disagreement is just a matter of having different points of view.)