About 20 years I reckon.

The 10 year point is where cars go from viable proposition to 'run it as a shed and only replace what's nescessary for the MOT'. Between 10 and 20 years old is when the value dips the most I reckon.

From 20 years on, the values start rising, and cars are worth working on again, so larger items failing (gearboxes, engines, PAS pumps etc) are suddenly worth reconditioning. At this point, the value attached to mileage drops off.

Also at about 20 years old, stuff is wearing out no matter whether it has been used or not. Rubber seals will be deteriorating, hydraulic components will be failing, bearings will need regreasing etc etc.

Part of what makes mileage less significant to value is the amount of cars available. If it's easy to find a 50k mile example of a car versus a 250k example then I think a lot of people will do that.

But as everyone has said, condition, condition, condition.

I don't see mileage being an issue so long as it has a comprehensive file of jobs that were completed telling you that the car has been maintained correctly.
I think you could possible have a classic that was 40 years old with 200k on the clock. I know of E30 M3s that are almost 30 years old and have 200k on the clock but you wouldn't be able to tell by the condition of the car because they've been looked after.

It depend on the car.

On something newer, say an E30 M3, it makes a massive difference to values.

On older kit, such as E-Types, it matters less as most will be totally rebuilt.

The GTA series are not going up in value any time soon though

The GTA series are not going up in value any time soon though

They're not going down to anywhere near what people thought they would. The earliest cars are 10 years old now: if it were a normal 156, it'd be £1500, tops. But they're hovering at around what, £4K and haven't seemed to drop any lower. Seems that being amongst the last "shove a big engine in it and hope foe the best" cars, and one of the more entertaining Alfas before the world went all MPG and emissions mad, has put in on the fast track to classic.

I wouldn't bother, good car but future landfill.

Classic cars gain value by being unusual and delivering something that nothing else manages. The 328i is a good car but the M3 is the one people will want, even then, the E36 is overshadowed by the E30

As with James, I bought my car to drive and at the same time have spared no expense on maintenance.
I love TVR's and was lucky enough to get my hands on a 2006 Mk2 Tuscan convertible, with only about 83 made.
I expect to change shocks etc. having already upgraded them. A rebuild for the engine will come along and other things like interior refurb and deepest.
If I look at the Tuscan market, higher mileage Tuscans with rebuilds are more valuable than lower lineage cars with no rebuilds. Yes, this is because of the fragile early engines but I think it relates to the value of any higher mileage older car s well.

It's played on my mind as I look at Tuscan prices rise but continue to put miles on mine but I want to enjoy.

Whilst many people say buy on condition and not mileage, mileage is still there psychologically. I also think the point when mileage becomes almost irrelevant will l depend very much on what are the potential issues with the specific car. If you consider a TVR then the cost of an engine or gearbox swap or rebuild is probably less than a body off chassis rebuild certainly for rv8 or v6 versions. Therefore if I was looking at a high mileage S, chim or griff I would be more interested with the chassis. If the mechanicals had 200k on but were performing well, either due to exceptionally good maintenance or having been refreshed and the chassis solid or replaced then I'd be happy. Given the supply of lower mileage cars though, you would still expect to pay less but probably not a huge differential.

When it comes to other vehicles then knowing the pitfalls I would think is key. I would imaging that a modern classic Ferrari an engine/gear box transplant would be very costly and hence mileage become more significant. How this relates to the GTA I have no idea. I can certainly see the 156 and 156s/w becoming moderately desirable and values becoming stable and as attrition takes place creeping up. Not everyone ants a BMW.

Some cars, mileage is ALWAYS going to be massively important.

To the Ferrari and Porsche crowds (GT3 RS and Scuderia) types, the fanatics get the arse ache and call a car leggy when it's over 15,000miles. Can't see that ever changing.

Some of the people I've met who are well heeled enough to be in the exotic classics circles are so particular and picky it's unreal.

My E-type has covered an indeterminate mileage since I have no idea if the odometer was right when I bought it. I don't think anyone can be sure of that given the mists of time. In my ownership we have covered 120,000, but she is sound as a pound bodywise, starts on the button and uses less oil than many a modern rebuilt engine. Strangely, it gets a lot of atention ebcause it does show signs of usage, call it patina or whatever, especially inside, and attracted one man's attention last year that he tried hard to buy it on the spot. Besides, classiccars with nominal mileage crop up from time to time at exhorbitant prices but I'd hesitate to own or drive one since the value is in the mileage and more importantly would the car be safe to drive without a full makeover?