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1) There is no immediate issue with age. USCIS does not discriminate against individuals that have a wide age difference.
2) However having said that, USCIS may flag this for acute review, meaning that they may be more careful about making sure that this is a marriage based on love, and not convenience. The interview process may be a little more in-depth. for example. However overall, provided that you can illustrate that the marriage is bona fide for love, then there should be no issues.
3) There is no difference in regards to (a) marrying in India and filing for a greencard then, or, (b) asking him to come to USA on a K1 visa and then marrying him here, and then filing for his greencard. However the latter (b) requires more paperwork since you'd essentially be filing for him TWICE - once for the K1 and once for the greencard; whereas the former (a) requires only one filing process - the greencard.
Both the K1 and the greencard are determined on the same basic facts - is the marriage bona fide? Ergo, it does not matter method you choose - it depends on whatever is more convenient for you.
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