Yes there WOULD be the same reaction if it were a man in his 30s dating a teenager, and yes a man of that age dating a teenager WOULD be thought of as immature. Like a pp, I also question the motives and scruples of someone in their 30s dating a teenager. It's grim. Actually grim.
And as a pp said, even though older women dating someone much younger - even young enough to be their son - are all smug, and 'ain't you all so jell of me?' I can assure you that most people are laughing at you behind your back (when they're not grimacing at the thought of it..) In addition I agree with previous posters that the friends and colleagues of the younger man will be taking the piss too. And his family will be horrified by it.
I do wonder WHY a woman wants/needs a man who is much younger? Maybe she is looking for a carer for when she's a bit older.
Also, these women assuming other women are jealous of them, do look a bit daft to be honest. No-one is jealous, there is no REASON to be jealous, and you're embarrassing yourself and coming across as a bit of an arsehole by claiming women are jealous of you because of your younger man.
All I feel is embarrassed for them, and a bit sorry for them, because they never have anything in common with these 'younger men' and the men often turn out to be cocklodgers and/or generally immature dickheads, and she often turns out to be like a mother to him. (Waiting on him hand and foot, and giving him handouts, and letting him live rent-free at her place.) I just feel sorry for any kids she has. Must be incredibly embarrassing and humiliating for them. 
I have known several women with men 15-20 years younger, and they get SO angry that people think she is his mother. Well of COURSE they will with an age gap like that. Hilariously, these women always think they look dead young. They don't. Like everyone else, they look their age. And so does he. And THAT is why people think you're his mother!
Regarding the hilarious 'aren't you jealous' bollocks... I know someone recently, who had her third son in the spring, and she sent pics of all 3 boys to her friend she had known for 20 years, (who has 2 daughters,) saying 'here's a pic of my 3 lovely iccle boys! thought I'd send it to make you jealous!' 
Just absolutely cringeworthy for this woman to think ANYone would be jealous of her. Especially as it's common knowledge that this woman wanted a daughter desperately, and cried for a week when her third child was another boy! 
I also knew someone who moved to Australia around 2013 who kept sending me pics of her house and garden (with a pool!) to make me jealous... I was confused and slightly annoyed, because I had no desire to visit Australia, let alone LIVE there. I care too much about my family and friends here to piss off to the other side of the planet and leave them all behind!
Then in 2016, she came back (with her DH,) and said the 3 years had been the unhappiest of her life, and they had lost around £50,000 during the 3-4 years (what with being there, and preparing to go - loss of income, loss on the property, travel costs, air fares, having to put a deposit on a private rental house in the UK to come back to. etc...)
Long story short, when someone is desperately trying to convince you that they are soooo happy, and you must be soooo jealous of them, they are almost ALWAYS not happy at all. They are trying to convince themselves AND everyone else that they are happier than ANYONE else, but most people can see straight through them! 
As has been said 'methinks the lady doth protest too much.........'