I've just had a quick look in the terms and conditions of the 16 - 25 railcard and this one surprised me a bit, the ones in red.
"4. Conditions of use of the 16-25 Railcard
4.1. You must be aged between 16 and 25 years old to purchase this Railcard, unless you qualify under condition 4.3.
4.2. 3-year 16-25 Railcards are available online only and may be purchased up until the day before your 24th birthday.
4.3. If you are a full-time student over 25 years old, you may purchase the 1-year Railcard with valid proof of full time study. See
16-25railcard.co.uk/maturestudents for details of the valid proof required.
4.4. If you bought your physical Railcard at a rail station, you must carry your separate 16-25 Photocard that was issued at the time of purchase (or where appropriate a ‘Permit to travel without Photocard’) and you must present this in any circumstance where you present your physical Railcard (i.e. when buying tickets and travelling with Railcard discounted tickets).
4.5. The 16-25 Railcard has a minimum fare that applies from 04.30 and 10.00 Monday to Friday. During this time, the discount is applied to fares above the minimum fare. This minimum fare does not apply to:
4.5.1. Advance tickets; or
4.5.2. journeys on public holidays; or
4.5.3. journeys during July and August.
At the time of printing, the minimum fare is £12. The minimum fare is subject to change during the validity of your Railcard - check website for the most up to date information.
INFORMATION:
- As long as you are aged 25 at the time you purchased the 1-year Railcard, you may still travel at the discounted fare throughout the validity period of the Railcard. Likewise, 3-year 16-25 Railcards purchased by the day before your 24th birthday will be valid for a full 3 years."
So it looks like you can get two more years out of the railcard if you buy it at the right time.