[1/19-20]
As how the divine sweets was distributed among the three queens, Raam is the first-born and thus the eldest. This is because the sweets - half of the whole vessel, was first given to Queen Kaushalyaa and She immediately ate it as an Empress. 

Sumitraa waited a while till Kaikeyee got her share. Kaikeyee got her portion but waited till her elder sister Sumitraa completed her drink. In the meanwhile Dasharath thought a while, and gave Sumitraa another one-eighth part. Sumitraa then consumed her two parts. Later Kaikeyee drank her portion. These timings of drink makes Raam the eldest one whereby he becomes eligible to be nominated as crown prince, in the later episodes.

Is Raam Full or Partial Avataar?

Raam's Age

[3/38]
Raam's age was less than 16 years, as Dashrath said to Vishwaamitra Jee, when he came to Dashrath to ask for Raam to take to protect his Yagya. [1/19-20] Maareech also confirms it while advising Raavan, that a boy of 12, Raam, killed my brother (Subaahu) and mother (Taadakaa) and hit me out [3/38]. It seems that Maareech lessened the age of Raam to intimidate Raavan.

The things like moustaches, beard, etc which are common to humans or to birthed beings but not to the mythological entities. They were not even there on Raam's face. As a young scholar, the boys are supposed to dress "simply", hence 'one-piece clothing...' is said. And this suggests that He was not wearing any shield or armor to keep Him safe. 

In Baal Kaand, at the outset of the journey of Raam and Lakshman with Vishwamitra Jee, it is said that they wore finger-covers made of certain lizard skin, which means that their fingers were very tender for using arrows. His mothers delicately arranged the locks of His hair, because He was still a 'boy' to them, and more so, he was with 'boyish' ornaments too, not verily embellished with jewelry. He appeared like some providential 'boy' hitherto unseen, with the height of a peacock, with the complexion of peacock-neck-blue, and moving about like a noble peacock... but that peacock was not screaming, yet His bow was twanging, leave alone the screeching and screaming.

Vishwaamitra Jee took Him only for 10 days, and in the meantime he was married to Seetaa also. So at the time of His marriage also He was 16 years old.

He must have been of 25 years of age (12+13=25) or 28 yeras of age (16+13=29) when He was exiled as Seetaa said so   ...  "we enjoyed royal pleasures for 13 years."

DOUBT: 
(1) Raam was 16 years old at the time Vishwaamitra Jee took Him. 
(2) Since he took Him only for 10 days, He was married at the same time: at the age of 16. 
(3) They lived happily for 13 years in Ayodhyaa (16+13=29). 
(4) Then they came to Chitrakoot and lived there for 2 years (29+2=31), 
(5) Then thy came to Dandak Van and lived there for 10 years (31+10=41). 
(6) Then they came to Panchvatee and lived there for 2 years (41+2=43). Seetaa was abducted in the beginning of this period itself. Maareech says to Raavan - "When I had thousands of elephants strength" at the time when he attacked Vishwaamitra Jee's Yagya. This shows that very little time, only 25 years, passed to the previous event. 

Did Maareech get so old in 25 years only? (41-16=25) that he lost his so much strength?