Why not the infantry or any arm/trade? If the individual can do the job why not? We all know the old and bold who can still hack it, beating the new guys and girls on PT, vast amounts of experience etc etc.. I know that promotion would slow but if you do the full time you'll get there in the end, after hard work and deserving it.
I've known plenty of guys who were running rings round the youngsters the day before there 22 year point and i know who i'd rather work with!