Hi,
You are not at all too old at all, but then I would say that ;-)
I've done option 3, started a Maths degree at age 37 and have just graduated at 44. I have a conditional offer (which I should be fine with) to do a full-time MSc in Oceanography, which was my long-term goal. I've worked for 20 years in IT as a programmer and can't wait to leave!
Advantages:
* You get to see if you enjoy the subject (OU have a B.Eng (Hons), which I'm sure you've looked at), and actual study, without committing yourself to giving up work, and get to pay piecemeal. I can only speak for my subject, but in general the quality of materials and support was excellent, and I really enjoyed the course as a whole.
* Studying while working is a nice compromise - you don't give up your salary, and I found that studying helped get me through doing a job I did not love.
* My impression is that OU students are well-regarded for post-grad studies, as you have already proved your self-motivation and ability to work unsupported. If you're prepared to give up a successful career to study something, then you are pretty committed.
Disadvantages:
* the cost of OU courses has increased massively (I was on transitional arrangements which fixed the module costs at the old prices. Not sure I could afford to do it now...).
* Studying and working full-time is *hard*, especially if you have a family. Loads of people do it every year, but it does take effort. Although the level 1 and 2 courses are easier, level 3 are pretty demanding.
* It will take you 6 years, most likely, so you wouldn't be doing a Masters until your are 41 or so. But still another 20+ years of work ahead of you, possibly!
* I don't know how suitable the OU Engineering degree is for a Civil Engineering masters. Best bet is probably to contact the applications office for any courses that appeal and ask.
Feel free to ask any questions or DM me...