The youngest a horse can race in the UK is as a 2 year old. All horses birthdays are considered to be 1st January so dependent upon the actual date a horse was born there can be a considerable age difference between 2 year olds. Those born early in the year will obviously be more advanced than the later born.
However, trainers will not allow their 2 year old horses to run until they consider them strong enough and ready to run. Early in the flat season these races races are restricted to 5 furlongs,(1100 yards) and as the season progresses this is increased to 6 and then 7 furlongs and finally up to 1 mile.
As a punting medium, betting on juvenile races is not recommended, especially early in the season. The problem is that most are having their first or second runs and you know very little about them. There are some trainers who are experts at producing juveniles ready to win at the first time of asking. Equally many trainers prefer to give their horses a gentle introduction to the race course. So they discourage the jockeys from giving the horse a hard first race.
Betting markets for 2 year old horse races tend to thrive on reputation and rumor. Any youngster coming from a top stable is regarded as a good bet and the hint that a horse looks “promising” is enough for it to start odds on. I have not tested it but it would not surprise me to discover if you had a lay bet every odds on 2 year old with Betfair you might show a profit.
In August the 2 year old handicaps start and this really can be a minefield for the punter. In his excellent little book “Form Book Analysis” M L Midgley suggested in 2 year old races look for a horse that starts as 2nd to 4th favorite but then creeps into favorite or near favorite spot. His think was that this would indicate either stable and or “inspired” money was being put on the horse. I think that this small book may now be out of print but if you can get hold of a copy it is well worth reading.