spew my dd is 6, but we have had a family railcard since she was 8 weeks - if two adults are travelling then it is nearly always cheaper to use the family railcard, even though you have to buy a ticket for the underage child (does mean they have the right to a seat too, which can be a blessing at times, particularly if you've reserved it, so it is clearly 'yours').
with a family railcard adults save 1/3 of the journey price, children save 2/3. you can also use it for advance bargain fares, making them even more of a bargain.
Even on a short journey, like coventry to birmingham it is about 20p cheaper - obviously not worth buying the railcard for, but if you have one anyway then worth using.
we got our first one because we were going on holiday on the eurostar and needed to get a train to london - it was worth buying (full price too!) just for that one (return...) journey. nowadays, we like to arrive on holiday by bedtime, so catch an early eurostar and a correspondingly earlier train to london - with a family railcard you can travel at peak times on an off-peak return.
i'm not trying to make you buy something you don't need
, i'm just pointing out that you could save money where you think you don't need to. unless you never travel by train. in which case forget it!