I think 36 is mature, and have advertised myself as mature since I started doing this in my mid-30s.

I genuinely don't understand why you're so put out. Being called mature has nothing to do with age per se, or how you look, it's about having life experience - this is what he's looking for, someone with a little bit more about them than just youth, a woman who knows who they are. An ease in oneself that comes with age, something many men like and often cite as a reason for seeking out a mature escort. In essence, he's looking for a woman rather than a girl. I think that's a compliment.

If he would have called you old, then that's a whole different matter, but he didn't, he chose his words with care , plus he opted for someone who he felt was age appropriate. In my book, this is actually all good.

But, irregardless of what he said, you found his tone creepy - reason enough to decline him.

I recently received an email from a man in his mid 60's saying he'd like to meet me as I'm mature. I'm 36!

I know I'm not 20 anymore, but I thought this to be a bit extreme. I would class mature escorts as anyone over 50. He said he wouldn't like anyone young enough to be his granddaughter, but still feel a bit put out, especially as I look young for my age.

His tone was somewhat creepy, so I've refused to meet him anyway. It is not a good thing when initial comms are like this 

I wouldn't take it to heart Gypsy,  after all, mature just actually means fully grown , either physically or in terms of attitude and behaviour, it doesn't mean old in terms of age.
He did say he didn't want someone young enough to be his granddaughter so I suppose technically at 36 you fit that criteria.
You see, you say you regard mature as escorts 50+ but in terms of escorting I have 40+ in my head, don't know why really lol, so its very subjective.

Mature in this industry seems to be anything above 30 years of age but this is not the real world. I even have some clients thinking mature is 28 years onwards.

Mature has multiple meanings, it does mean full grown, but it also means advanced in years. Its a pretty arbitrary concept, ultimately, used in different ways.

I'm just a bit younger than you, and though I wouldn't be offended, I'd find it a bit silly if a man almost twice my age referred to me as 'mature', seeing as I'd be pretty young relative to him. Just like I find it a bit silly when men older than me call me a MILF (but in porn I think the average age performer in this category is about 26/7  Though I don't think its unreasonable to have been irked, 'society' ages women more speedily than it does men. I remember the farago about actress Maggie Gyllenhaal at 37 being told she was too old to play the girlfriend of a 55 year old actor in a film...

Like I say its relative; if some 22 year old client called me mature, then fine. If some 60 year old guy did it  to me, he'd get the narrow eye treatment.

Mature has multiple meanings, it does mean full grown, but it also means advanced in years. Its a pretty arbitrary concept, ultimately, used in different ways.

I'm just a bit younger than you, and though I wouldn't be offended, I'd find it a bit silly if a man almost twice my age referred to me as 'mature', seeing as I'd be pretty young relative to him. Just like I find it a bit silly when men older than me call me a MILF (but in porn I think the average age performer in this category is about 26/7  Though I don't think its unreasonable to have been irked, 'society' ages women more speedily than it does men. I remember the farago about actress Maggie Gyllenhaal at 37 being told she was too old to play the girlfriend of a 55 year old actor in a film...

Like I say its relative; if some 22 year old client called me mature, then fine. If some 60 year old guy did it  to me, he'd get the narrow eye treatment.

Yes, you're absolutely spot on there 

I struggled to market myself in my 30s. It got easier 40s and 50s

Me too. 30s is a kind of in between age for me. You're not "young" but you're not into real "milf" territory. It's alright for men, but as women we're keenly aware of declining fertility rates and the message that brings us (ie you're no longer fertile so you're useless.)

I'm much more comfortable in my late 40s and post menopausal. I now get trade from blokes my own age, plus older guys who don't want to fuck someone their daughter/grandaughter's age, who want someone with a bit of conversation, someone understanding of issues with mobility/ED etc. Plus I also pick up the younger guys with the "my best friend's hot mum" fetish.

I'm 35 so I'm wondering if I'm classed as mature.
And if I did, can i advertise myself as mature and pregnant?

I'd say 35 is mature but opinions differ as you can see.
Why not if you are?