If he had enough time to have trained his starter to the point it died (I always assumed the illness had been contracted due to tiredness), grieve and wander about without a aim, plus he still got another team of strong Pokémon... so that alone indicates he cannot be very young, even if he started his journey when he was 10. Then there are the wrinkles, but honestly I've seen plenty of men in their middle 30s looking like that because they work outside. Alder himself seems to be the type of man who enjoys a good journey so... I would put him between 36-38 years old.