Found it! Abhimanyu is indeed older than all the Upa Pandavas. Subhadra becomes pregnant almost immediately after her marriage.
https://www.sacred-texts.com/hin/m01/m01224.htm
SECTION CCXXIII
(Haranaharana Parva)
O Bharata, the high-souled Vasudeva remained there with Arjuna in the delightful city of Indraprastha. And the illustrious one wandered over the banks of the Yamuna in search of deer. And he sported with Arjuna piercing with his shafts deer and wild boars. Then Subhadra, the favourite sister of Kesava, gave birth to an illustrious son, like Puloma's daughter, (the queen of heaven) bringing forth Jayanta. And the son that Subhadra brought forth was of long arms, broad chest, and eyes as large as those of a bull. That hero and oppressor of foes came to be called Abhimanyu. And the son of Arjuna, that grinder of foes and bull among men, was called Abhimanyu because he was fearless and wrathful. And that great warrior was begotten upon the daughter of the Satwata race by Dhananjaya, like fire produced in a sacrifice from within the sami wood by the process of rubbing. Upon the birth of this child, Yudhishthira, the powerful son of Kunti, gave away unto Brahmanas ten thousand cows and coins of gold. The child from his earliest years became the favourite of Vasudeva and of his father and uncles, like the moon of all the people of the world. Upon his birth, Krishna performed the usual rites of infancy. The child began to grow up like the Moon of the bright fortnight. That grinder of foes soon became conversant with the Vedas and acquired from his father the science of weapon both celestial and human, consisting of four branches and ten divisions.
"Endued with great strength, the child also acquired the knowledge of counteracting the weapons hurled at him by others, and great lightness of hand and fleetness of motion forward and backward and transverse and wheeling. Abhimanyu became like unto his father in knowledge of the scriptures and rites of religion. And Dhananjaya, beholding his son, became filled with joy. Like Maghavat beholding Arjuna, the latter beheld his son Abhimanyu and became exceedingly happy. Abhimanyu possessed the power of slaying every foe and bore on his person every auspicious mark. He was invisible in battle and broad-shouldered as the bull. Possessing a broad face as (the hood of) the snake, he was proud like the lion. Wielding a
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large bow, his prowess was like that of an elephant in rut. Possessed of a face handsome as the full-moon, and of a voice deep as the sound of the drum or the clouds, he was equal unto Krishna in bravery and energy, in beauty and in features. The auspicious Panchali also, from her five husbands, obtained five sons all of whom were heroes of the foremost rank and immovable in battle like the hills. Prativindhya by Yudhishthira, Sutasoma by Vrikodara, Srutakarman by Arjuna, Satanika by Nakula, and Srutasena by Sahadeva,--these were the five heroes and great warriors that Panchali brought forth, like Aditi bringing forth the Adityas. And the Brahmanas, from their foreknowledge, said unto Yudhishthira that as the son of his would be capable of bearing like the Vindhya mountains the weapons of the foe, he should be called Prativindhya. And because the child that Draupadi bore to Bhimasena was born after Bhima had performed a thousand Soma sacrifices, he came to be called Sutasoma. And because Arjuna's son was born upon his return from exile during which he had achieved many celebrated feats, that child came to be called Srutakarman. While Nakula named his son Satanika after a royal sage of that name, in the illustrious race of Kuru. Again the son that Draupadi bore to Sahadeva was born under the constellation called Vahni-daivata (Krittika), therefore was he called after the generalissimo of the celestial host, Srutasena (Kartikeya). The sons of Draupadi were born, each at the interval of one year, and all of them became renowned and much attached to one another. And, O monarch, all their rites of infancy and childhood, such as Chudakarana and Upanayana (first shave of the head and investiture with the sacred threads) were performed by Dhaumya according to the ordinance. All of them, of excellent behaviour and vows, after having studied the Vedas, acquired from Arjuna a knowledge of all the weapons, celestial and human. And, O tiger among kings, the Pandavas, having obtained sons all of whom were equal unto the children of the celestials and endued with broad chests, and all of whom became great warriors, were filled with joy.'"
Edited by RamKiSeeta - 5 years ago