To your title: No, its not quite normal. Depends what kind of feral cat it is, really.
Feral doesn't equal feral. Cats can be strays but they still have a past, f.e. this one maybe had some positive experiences in their formative months and therefore isn't completely opposed to human contact now. Plus, of course, you seem to be doing a good job :)
I would try not to pet them on your own accord, but offer your hand instead. They might rub against the hand and show you how they like to be petted, or they might get skittish which will tell you that 'hands' are a touchy (ha) subject still. To get a feral cat to associate hands with good things, you can try giving treats/food closer and closer to yourself until the food can be eaten out of the hand, the rest will come with time and patience.
Now to your other questions.
Since the cat is allowing me to pet him. Is there a way that I can
attract it to live/sleep in a better place?
Yes and no. You can give them options, f.e. a box with styrofoam (insulation) inside and maybe hay for warmth. You can also somewhat move their feeding spot bit by bit and therefore establish a new place as 'safe' or good. But ultimately, they will go where they wish...
Also, how can I estimate it's age?
Usually this is done by checking th cats' teeth, but even that only gives a very rough estimate (kitten, youngling ~1, 1-4, 5-10ish, anything above). Are they castrated? If not, that should definitely be a thing on your to-do list. If they are, then the vet should definitely check for a chip/tattoo.
I want to get some blankets for it but not sure how to let him
Not sure I understand this question - just put down some blankets and maybe put some treats on top :) However, I would advise against using blankets outside, as they get damp very quickly. Styrofoam and Hay is a lot better, though the latter has to be changed every now and then as well.