The 36.2 300m wasn´t THE FOLLOWING YEAR (to 1973) but one year later namely 1975 (he was injured most of 1974). The junior international 1500m win was also in 1975 where Coe 18.
Coe started early and was an good runner on local and regional level from early on but he wasn´t dominant on national and international level before passing 20.
In his youth wasn´t a talent comparable to the current young top runner Jakob I.
Age 13:
Coe: 4:31 in the 1500m
Jakob: 4:05
Age 14:
Coe: 2:08 in the 800m and 4:18 in the 1500m
Jakob: 1:52.60 and 3:48.37
Age 15:
Coe: 1:59.9, 4:05.9 and 8:49.9 in the 3000m
Jakob: 1:51.07, 3:42.44 and 8:22.25
Age 16:
Coe: 1:56.0, 3:55, 8:34.6
Jakob: 149.4, 3:39.92 and 8:00.01
Age 17:
Coe: Injured
Jakob: 3:31.18 in the 1500m
Age 18:
Coe: 1:53.8, 3:45.2 and 8:14.2
Jakob: 3:30.16
Age 19:
Coe: 1:47.4, 3:58.3 in the mile
Jakob: 1:46.44 and 3:51.30 in the mile at 18 (didn´t run at 19)
So until age 20 Jakob was on a much higher level than Coe except for the 800m where Jakob was only about 1 second in front at 19.
Coe got a bronze in the European Junior Championships at 18.
Jakob didn´t win a 1500m U20 title since he was tripped in the final at 16 but he won the 5000m and the 3000m st. easily. And next year at 17 he took silver in the U20 WC 1500m and bronze in the 5000m. And later that year he won the European senior title in both the 1500m and the 5000m.
Coe ran a lot of X country races as a young teenager but was normally beaten on the national level.
Jakob won at least a Nordic title as a young teenager.
And at age 15 he was a close runner up in the national SENIOR X country 4 km race and after that probably nobody in Norway (including his older brothers?) were able beat him.
Shortly after turning 16 he won his first European U20 X country title which was followed by 3 more (until he moved directly to the senior level where he now has 2 titles).