Depends what gig you are after when you finish your training. If its a private jet or niche operator then fine. Only proviso is that you are aware of the money on offer over a career and the often 24hr on call basis that these jobs bring.
If its an airline then think long and hard. Firstly getting into an airline as your first job when you are late 30s or 40s is tough enough. Airlines like young keen individuals that are unlikely to rock the boat. A family person who has a previous career is unlikely to fulfill this criteria. Think CTC and the lucky few that are getting selected into the current cadetships at BA and Aer Lingus.
But if you are in the lucky few that do jump through the hoops and beat the odds then there is a long seniority system ahead of you.. If its a loco then you probably wouldn't have gotten the job in the first place (allegedly if T&E is to be believed). Starting at 40 you will be around 1-2 years in training making you say 42 when your licenced to thrill. Time to command would be at a minimum of 6-8 years in most operators. In ours its something like 10-12 for a short haul command. 6 years minimum before you will see the RHS of something bigger than an A320/1.
Then there are the medical issues which unfortunately as we get older become more statistically likely to hit us.
Think long and hard before putting your family through all of that and come home with a roster without any weekends off and Xmas away from the kids.