I don't think you'll find an orchestra that will allow you to learn 'on the job', so you'll need to get some lessons first. My son learns orchestral percussion at school and I think with some basic musical knowledge you could reach a useful standard quite quickly. He started last year, has Grade 4 piano, and is now working towards Grade 4 on percussion.
The big problem to my mind is: where do you get your timps? I imagine most percussionists don't own a full set, but that they're owned by the orchestra.
One thing you could do to get started is have a look for education/outreach programmes by the major orchestras - many of them run taster days, workshops, etc. that are open to all. Try the Royal Philharmonic's website, for example. The BBC Proms also run family workshop days and you don't need any experience. Once there, get talking to whoever's in charge of percussion and ask for advice.
I don't think all amateur orchestras ask for Grade 8, especially for unusual instruments. You just have to keep searching! If you're in a bit city then obviously there will be more choice.
You could also try posting your question on the ABRSM forums (Associated Board of the Royal Schools of Music). Don't post in the percussion bit, as it's very quiet; try one of the busier sections, like the Café. They have a useful list of amateur musical groups here. Also try the Local Amateur Music Network
Meanwhile, you could start brushing up your piano skills so you remember all you knew about reading music etc.
And no, speaking as a late-flowering clarinettist, 40 is not too old!! The AB forum is full of inspiring people who have started much later than that and done great things.