We have to remember that Sri Rama and Lakshmana are sons of same king Dasaratha but born to 2 different mothers, ie., Kausalya and Sumitra.
Whereas, Balarama and Sri Krishna are brothers of same parents, Vasudeva and Devaki. However, during the pregnancy, the embryo of Balarama had been transferred from the womb of
Devaki to that of Rohini, the eldest wife of Vasudeva.
Sri Rama was born on Punarvasu nakshtra, Bharata on Pushymi, while Lakshmana and Satrugna were born in Āshleshā Nakshatra.
According to Ramayana, Sri Rama was born on the Ninth day of Chaitra month, whereas Lakshmana was born on later part of 10th day of Chaitra month.
So age difference between Sri Rama and Lakshmana is maximum of 2 days only.
In respect of Balarama , the embryo was transferred from Devaki to Rohini. After that Devaki conceived again and gave birth to Sri Krishna.
Hence, at least 1 year gap will be there between the birth of Balarama and Sri Krishna.