According to the captions of the first Thor movie, the battle between the Jotuns and Asgardians take place in Norway, 965 AD. Around this time, Loki was born.
In Thor 2 the life expectancy was stated to be around 5,000. The average human life in developed countries from what I’ve gathered is approximately 82.
Therefore, in human years Loki is somewhere around 17.
Chris Hemsworth did call Thor in the first film a teenager, so he knew what he was talking about.
Thor can’t be older than Loki more than a decade tops, otherwise he would have remembered that the baby came out of nowhere. No pregnancy etc.