Well I have no kids but I was in your son's shoes 30 years ago so I'll jump in and say a word

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I believe first time I did the full Pascha Lent I was probably 11 or 12, while I'd been doing the Wed and Fri fast basically since I remember myself. If I remember correctly, I think both mum & I basically wanted to experiment & see how long I would last with the Pascha Lent

. My older siblings & of course my parents were already doing it. I don't recall being pressed to fast (or to do anything else, for that matter). I have never been too strict (meaning, I do oil & wine on fasting days except very few days per year, but the rest is normally out).
I think you'll do great to get your son into the habit of fasting (
health permitting). Decades later, I will tell you that for me it's the one & only "churchy" thing that's very easy to do - it's pretty much automatic. Otherwise, as the years go by, I do find it increasingly difficult to get my act together and keep up with basic things such as a simple rule of prayer, or even consistently going to church on Sundays at a sensible hour

. What fasting often does, it reminds me that, you know, I actually believe in God - when often these days there's nobody and nothing else at all to remind me of just that, and my own self is doing his best to make me forget it. Looking back in the last 10 years or so, I think I would probably have given up altogether if it wasn't for such small habits as not milking my coffee on fasting days to ring the bell for me. Really, when temptations, disappointments, tiredness, sloth, whatever strikes, it's through small habits that God begins the process of bringing us back.
Also, from another perspective I don't think it's good for you at all to have meat 365 days a year... Staying away periodically is actually a healthy habit. Beans, lentils, chick peas, aubergines, spinach etc are all healthy, tasty & nutricious & I don't think kids (or adults) miss out on energy if they eat them instead of meat twice a week. I mean, I have been fasting all my life & I am not malnourished. I am not short & I could be thinner

. In primary school I was maybe the only one in my class doing any sort of fasting and yet I was probably one of those typical fat boys that every primary school class has

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These are general thoughts. Implementation details - at what age, how etc - I'm not a natural or a spiritual parent so I can't say anything on that

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I actually often miss the okra that mum used to cook

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