Raising the NHL's draft age would be the best thing for the league because it would create more interest in the players selected and the teams selecting them. Imagine what it would be like if the players drafted in the 1st round were all expected to have an impact on their new teams in the upcoming season.
That's what happens with the NFL's draft and it's no coincidence that the NFL's draft is probably the most watched and most closely followed in American sports (keep in mind I'm writing this from a US perspective). Baseball and hockey's drafts are non-events to most fans - even most fans of those sports - because players get drafted and it takes years for most of them to develop.
One idea I kind of like that would have some of the same effects of raising the draft age, while actually allowing the more talented 18 year olds to get drafted, would be to require that all 18 year old draftees remain on a team's major league roster until they reach the age of 20 or 21. If a team wants to demote such a player then they would have to put him through waivers and risk the likelihood that he'd be claimed by a weaker team.
Under such a system, there would be the obvious interest generated when a team drafted an 18 year old because fans would see that player in their favorite team's uniform starting next season. Players drafted over the age of 18 would also mean more to fans because there's a greater likelihood that they would have more immediate impacts on the fortunes of teams drafting them.
However, that's not the only benefit to such a system. Stronger teams in the league would probably not draft that many 18 year olds because they wouldn't want that many inexperienced players stuck on their big league roster for the next 2 or 3 seasons, while they would be trying to make playoff runs. If a stronger team did keep an 18 year old on their roster, it would mean that a player who would otherwise be on their roster would be available to another team, which would help spread the wealth regarding some more-proven players throughout the league.
Such a system would obviously provide weaker teams with older draft picks who were closer to being NHL-ready and could help the NHL team sooner rather than later but, just as importantly, such a system would also leave rebuilding teams with the ability to draft more 18 year olds and they could stock their roster with a greater number of higher quality 18 year old picks. Under such a system a team that went into rebuilding mode could really turn things around in 3 years.
Of course, you would have to limit the amount of 18 year olds allowed per team so you didn't have a junior team out there on the ice against NHL competition and I'm sure there's other pitfalls I haven't thought of but I think such a system could go a long way to creating more interest in the draft and helping the weaker teams get more competitive, faster.