The "spark of hunger," that the speaker mentions is another way of phrasing determination.
Ambition is being personified as a physical object which is the pilot light, in a more technical term it means that its a small spark or flame that is used to ignite a larger, more powerful gas burner. (Example: when you use a match stick to light a firework)
Though the speaker makes sure to describe the pilot light as being faulty, this could be understood, from our example, as trying to use a match stick from a box that hasn't been used in years. When you finally bring out the box of match sticks and try to use it, the matches aren't the same, as the years passed the matches became more brittle to where some will work but not to it's full potential. When you finally light the match, it sparks on but the spark dies out just as quick instead of a newer match that stays lit for a few more solid seconds.
As one grows older they see and face various hurdles but as they gain experience, it does come at the cost of losing their will to be positive and keep moving forward so when compared to a younger individual that just left home for the first time, excited about what the bigger world will give them, ready to face head on any hurdles, they're determination is limitless. The new person would be the new match versus the older person being the old match.
If you need a more in depth and detailed explanation, reach out and I'll be more than happy to help! You've got this.