According to Ashrama system, there are four Ashrama which are Brahmacharya, Grihashta, Vanaprashtha and Sanyasa. The Karma Kanda part of Veda i.e Brahmanas are meant for Grihashta whereas Jnana Kanda part of Veda i.e Upanishads are meant for Sanyasa.
The primary topic of Brahmanas (Karma Kanda) are - Yajna and rituals. And the primary topic of Upanishads (Jnana Kanda) are: Spirituality.
Now, Aaranyaka is considered as bridge from Brahmanas to Upanishads.
In Brahmanas, the concentration is on Yajna. Aaranyaka introduces subtle thinking in (inner) Yajna-Anushthan and leads towards spiritual angle rather than that of Sakama Karma in (outer) Yajna-Anushthan. This helps in Vanaprashtha Ashram to lift from bondage of Samsara and provides spiritual preaches.
The spiritual philosophy we found in Upanishads are actually started/initialized in Aarnyakas.
Prana Vidya and Pratik Upasana are prime theme of Aaranyaka.
Thus, according to the topic/theme of Aaranyaka (bridge between Brahmanas and Upanishads), it is meant for Vanaprashtha Ashram.
When one comes out from social responsibilities, he is introduced the true sense of (inner or subtle) Yajna and preached the various Upasanas so as to lead to Upanishads. Also note that Brahmacharya is also expected for reading Aaranyaka which differs from Grihashta Ashrama.
Note: Outer Yajna means Dravya Yajna performed by things or objects to fullfil physical desires whereas Inner Yajna means Subtle Yajna performed by mind and intellect to improve spiritual level.