I was in a club with Connor Dunne and Alex Dowsett who both turned pro and my daughter raced a couple of pro races ( Fleche Wallonne and Tirenno adriatica ,unpaid though)
Connor and Alex basically won loads of races as a junior and also knew the right people , Connor is related to ex pros and Alex was mentored by a ex pro. They were both brilliant amateurs but also knew the right people. Amateur races for youngsters are mainly Crtiteriums where you get National points, once a junior you can race senior races including road races.
If a good Youth you can carry over points and when you are a junior which counts as a senior in road points you can start as 3rd car straight away and work your way to 1st cat.
but cannot reach elite status even if you get the points to do so.
My daughter got noticed as a 14 yr old by British cycling by winning at crit and CX events and got put forward for their race school programme and got invited to training weekends etc run bt BC . She also was a National Junior 25 Tt champpion and second in the 10 , she was also was the Eastern region senior womens crit champion when a first year junior. The word gets round and she got signed by a womens National team, being a womems team actually only one of their riders got paid, they did get teams frames , wheels and clothing but had to provide their own , bars, brakes and gears. She gave up as she had a old injury from horse riding which meant she go not train hard enough to go any further.
BC run a programme for youngsters, Cavendish and several others went through it, mainly track based. Loads get put through at the 14-16 year old level but at junior level they can go to a olympic programme when my daughter was up for selection to go to the olympic programme, BC decided that year only 2 women would go through , 6 men did though. This is based on what BC think their potential is, ie if none look they will get a medal then they pick none. May have changed now though.
I have know a guy who started in his early 20's and basically worked his way up through the road cat system to elite then got signed by a continental team. Took him around 4 years . His job was as a fitness trainer so got the time to train, also helped he again networked his way with people he met and knew .