Possible Clues:

• One slow to develop
• One developing at their own rate, or a hint to the last few letters of 18-, 31- or 50-Across
• One who catches up eventually ... or a hint to the ends of 17-, 30- and 46-Across
• Slow-to-mature sort
• *Slow-to-develop sort
• Delayed developer (with an up-front charge)