One major branch of Atharvaveda is said to be composed by Atharva rishi. He is son of Angirasa rishi and also called Atharvangirasa. Purans say that Atharva existed in the Swayambhuva manvantara.
As also, Atharvaveda is sadi to be the joint compostion of Bhrigu and Angirasa. And one composer was present earlier than Vaivasvat Manu and other composers of Rigveda. Hence Atharvaveda can be older than Rigveda.
Also if we study the topics these two vedas deal, we see that Atharvaveda is a very crude Veda which deals more with sacrifice, magical healing and black magic whereas Rigveda seems a bit more mature dealing with praises of gods and a part of history ( War of ten kings,etc) . So by this sense, Atharvaveda can be of an earlier era when less maturity about physical and meta-physical world existed. But we see that Bhrigus were then cornered and their relative verses in the Rigveda are quite less while that of Angirases are quite high. Some attribute that Bhrigus crossed sides and became the purohits of Asuras in Central Asia and later on again came into the Vedic fold. This also explains why Bhrigus have more hymns on Soma in Rigveda than any other family as Soma plant is found more in Central Asia.
i am quoting Shrikant Talageri who feels this way