I'm not familiar with the specific situation in the UK, but age-based discounts can support many policy goals.

Maybe you want teenagers to be more independent, instead of relying too much on their parents for transportation needs. Maybe you want young adults get used to using public transit instead of driving everywhere, which can lower infrastructure costs in a densely populated country. Maybe you want to encourage retirees to get out and participate in the society, instead of sitting alone at home. Or maybe you want to encourage the use of public transit outside peak hours, which could reduce the overall need for subsidies.

Simple systems, such as age-based categories, often work well enough. Targeted subsidies can be more efficient in principle. But that assumes that regulators manage to target them properly and have the regulations implemented in software correctly and in time. All of those often fail. And even when they are successful, they may cost more than you save with better targeting. Not to mention the opportunity costs: when regulators focus on one thing, they can't work on another.